# Join a marketplace campaign

Allow a creator to join a campaign from the marketplace.
This endpoint uses MarketplaceJoinabilityEvaluator for consistency with GET /marketplace/campaigns/:id.
Returns the created/updated participation with joinability snapshot.
Handles idempotency: joining twice returns the existing participation.
Requires authentication and creator role with verified email.

Endpoint: POST /marketplace/campaigns/{campaignId}/join
Version: 0.1.0
Security: bearerAuth

## Path parameters:

  - `campaignId` (string, required)
    Unique identifier for the campaign
    Example: "campaign_abc123"

## Response 200 fields (application/json):

  - `participation_id` (string, required)
    Unique identifier for the participation
    Example: "participation_xyz789"

  - `campaign_id` (string, required)
    Unique identifier for the campaign
    Example: "campaign_abc123"

  - `influencer_user_id` (string, required)
    Unique identifier for the influencer/creator user
    Example: "user_123"

  - `participation_status` (string, required)
    Current status of the participation
    Enum: "PARTICIPATION_STATUS_PENDING", "PARTICIPATION_STATUS_APPROVED", "PARTICIPATION_STATUS_REJECTED", "PARTICIPATION_STATUS_CONTENT_SUBMITTED", "PARTICIPATION_STATUS_COMPLETED"

  - `created_at_timestamp` (string, required)
    ISO timestamp when the participation was created
    Example: "2024-01-01T00:00:00Z"

  - `joinability` (object, required)

  - `joinability.canJoin` (boolean, required)
    Whether the current user can join this campaign
    Example: true

  - `joinability.reasons` (array, required)
    Array of reason codes if the user cannot join (empty array if can join)
    Enum: "NOT_AUTHENTICATED", "EMAIL_NOT_VERIFIED", "NOT_A_CREATOR", "CREATOR_PROFILE_INCOMPLETE", "NO_ELIGIBLE_INTEGRATION", "CAMPAIGN_NOT_ACTIVE", "CAMPAIGN_NOT_IN_WINDOW", "ESCROW_DEPLETED", "ALREADY_JOINED", "SUBMISSION_LIMIT_REACHED"

## Response 400 fields (application/json):

  - `error` (string, required)
    Error code that identifies the type of error. This is a stable identifier that can be used
for programmatic error handling. Common codes include:
- NOT_FOUND - Resource not found
- VALIDATION_ERROR - Input validation failed
- PERMISSION_DENIED - User lacks permission for the action
- CONFLICT - Request conflicts with current resource state
- UNAUTHORIZED - Authentication required
- INTERNAL_SERVER_ERROR - Unexpected server error
    Example: "VALIDATION_ERROR"

  - `message` (string, required)
    User-friendly error message that should always be displayed to end users. This message is
designed to be clear, actionable, and free of technical details.

This field is always present and contains the primary message for end users.
    Example: "The email address you entered is not valid. Please check the format and try again."

  - `technicalMessage` (string)
    Optional technical message for developers. Contains detailed information including IDs,
error codes, and technical context. Only included when it differs from the user-friendly
message field. When present, developers can use this for debugging while end users
should see the message field.
    Example: "Campaign with ID campaign_123 not found"

  - `details` (object)
    Additional error details (optional). May contain structured information about the error,
such as validation field errors or additional context.

  - `requestId` (string)
    Unique request identifier for tracing. Included in X-Request-ID header and can be used
to correlate errors with server logs.
    Example: "550e8400-e29b-41d4-a716-446655440000"

  - `timestamp` (string, required)
    ISO 8601 timestamp when the error occurred
    Example: "2024-01-01T00:00:00Z"

## Response 401 fields (application/json):

  - `error` (string, required)
    Error code that identifies the type of error. This is a stable identifier that can be used
for programmatic error handling. Common codes include:
- NOT_FOUND - Resource not found
- VALIDATION_ERROR - Input validation failed
- PERMISSION_DENIED - User lacks permission for the action
- CONFLICT - Request conflicts with current resource state
- UNAUTHORIZED - Authentication required
- INTERNAL_SERVER_ERROR - Unexpected server error
    Example: "VALIDATION_ERROR"

  - `message` (string, required)
    User-friendly error message that should always be displayed to end users. This message is
designed to be clear, actionable, and free of technical details.

This field is always present and contains the primary message for end users.
    Example: "The email address you entered is not valid. Please check the format and try again."

  - `technicalMessage` (string)
    Optional technical message for developers. Contains detailed information including IDs,
error codes, and technical context. Only included when it differs from the user-friendly
message field. When present, developers can use this for debugging while end users
should see the message field.
    Example: "Campaign with ID campaign_123 not found"

  - `details` (object)
    Additional error details (optional). May contain structured information about the error,
such as validation field errors or additional context.

  - `requestId` (string)
    Unique request identifier for tracing. Included in X-Request-ID header and can be used
to correlate errors with server logs.
    Example: "550e8400-e29b-41d4-a716-446655440000"

  - `timestamp` (string, required)
    ISO 8601 timestamp when the error occurred
    Example: "2024-01-01T00:00:00Z"

## Response 403 fields (application/json):

  - `error` (string, required)
    Error code that identifies the type of error. This is a stable identifier that can be used
for programmatic error handling. Common codes include:
- NOT_FOUND - Resource not found
- VALIDATION_ERROR - Input validation failed
- PERMISSION_DENIED - User lacks permission for the action
- CONFLICT - Request conflicts with current resource state
- UNAUTHORIZED - Authentication required
- INTERNAL_SERVER_ERROR - Unexpected server error
    Example: "VALIDATION_ERROR"

  - `message` (string, required)
    User-friendly error message that should always be displayed to end users. This message is
designed to be clear, actionable, and free of technical details.

This field is always present and contains the primary message for end users.
    Example: "The email address you entered is not valid. Please check the format and try again."

  - `technicalMessage` (string)
    Optional technical message for developers. Contains detailed information including IDs,
error codes, and technical context. Only included when it differs from the user-friendly
message field. When present, developers can use this for debugging while end users
should see the message field.
    Example: "Campaign with ID campaign_123 not found"

  - `details` (object)
    Additional error details (optional). May contain structured information about the error,
such as validation field errors or additional context.

  - `requestId` (string)
    Unique request identifier for tracing. Included in X-Request-ID header and can be used
to correlate errors with server logs.
    Example: "550e8400-e29b-41d4-a716-446655440000"

  - `timestamp` (string, required)
    ISO 8601 timestamp when the error occurred
    Example: "2024-01-01T00:00:00Z"

## Response 404 fields (application/json):

  - `error` (string, required)
    Error code that identifies the type of error. This is a stable identifier that can be used
for programmatic error handling. Common codes include:
- NOT_FOUND - Resource not found
- VALIDATION_ERROR - Input validation failed
- PERMISSION_DENIED - User lacks permission for the action
- CONFLICT - Request conflicts with current resource state
- UNAUTHORIZED - Authentication required
- INTERNAL_SERVER_ERROR - Unexpected server error
    Example: "VALIDATION_ERROR"

  - `message` (string, required)
    User-friendly error message that should always be displayed to end users. This message is
designed to be clear, actionable, and free of technical details.

This field is always present and contains the primary message for end users.
    Example: "The email address you entered is not valid. Please check the format and try again."

  - `technicalMessage` (string)
    Optional technical message for developers. Contains detailed information including IDs,
error codes, and technical context. Only included when it differs from the user-friendly
message field. When present, developers can use this for debugging while end users
should see the message field.
    Example: "Campaign with ID campaign_123 not found"

  - `details` (object)
    Additional error details (optional). May contain structured information about the error,
such as validation field errors or additional context.

  - `requestId` (string)
    Unique request identifier for tracing. Included in X-Request-ID header and can be used
to correlate errors with server logs.
    Example: "550e8400-e29b-41d4-a716-446655440000"

  - `timestamp` (string, required)
    ISO 8601 timestamp when the error occurred
    Example: "2024-01-01T00:00:00Z"