feat(social): enforce showcase ownership on updates (BA-056)

[ranjeetprabhatedu-dotcom]

Summary

This PR addresses issue #969 (BA-056) by enforcing project ownership checks on showcase updates, allowing only authenticated project owners to modify showcase entries.

• Changes:
  • BackendAcademy/src/social/showcase.service.ts: Added update method enforcing post.authorId === editorId with ForbiddenException for unauthorized callers and NotFoundException for missing showcases.
  • BackendAcademy/src/social/social.controller.ts: Added PUT and PATCH routes for showcase updates extracting author/editor identity.
  • BackendAcademy/src/social/showcase.service.spec.ts: Added comprehensive unit test coverage verifying owner edits, non-owner rejections (ForbiddenException), missing showcase rejection (NotFoundException), and updated URL validations.
• Verification:
  • All 18 tests in showcase.service.spec.ts pass cleanly (100% Green).
  • Scope strictly restricted to the 3 allowlisted files.

Closes #969