Reindex flow fails for existing repository URL (409 conflict)

[rycerzes]

• Details: Reindexing a repository using the same Git URL currently returns 409 Conflict instead of updating/reprocessing. Define expected behavior for:
  • same URL + same commit (idempotent no-op or forced reindex),
  • same URL + older/newer commit (allow reindex and refresh indexed data),
  • clear API response states for each scenario.
• Acceptance Criteria: Reindex endpoint supports same-URL requests without hard conflict when reindex is intended, and commit-aware behavior is documented/tested.