What kind of change does this PR introduce?

Feature - Publication lifecycle management and pipeline validation

What is the current behavior?

Fixes #41: Publications can be deleted while pipelines still reference them, causing runtime failures. Pipelines can be created with non-existent publications.

What is the new behavior?

Added validation and protection:

• Publications cannot be deleted if referenced by active pipelines (returns 409 Conflict with pipeline IDs)
• Pipeline creation validates publication exists on source database (returns 400 if missing)
• Pipeline updates validate publication exists on source database (returns 400 if missing)

New endpoint:

• GET /v1/sources/{source_id}/publications/{publication_name}/pipelines - Lists all pipelines using a publication

Updated endpoints:

• POST /v1/pipelines - Now validates publication exists before creating pipeline
• POST /v1/pipelines/{id} - Now validates publication exists before updating pipeline
• DELETE /v1/sources/{source_id}/publications/{publication_name} - Now returns 409 Conflict if pipelines reference the publication