feat(api): add tag-based revalidation to FrontendRevalidationService

[joaquimscosta]

Summary

The frontend now supports tag-based cache invalidation via cacheTag() (added in PR #85), but the backend FrontendRevalidationService only supports path-based revalidation ({"path": "..."}). Add a revalidateTag(tag: String) method to complete the end-to-end cache invalidation flow.

Current State

• Frontend: /api/revalidate endpoint accepts {"tag": "gallery"} and calls revalidateTag(tag, { expire: 0 })
• Frontend pages: Use cacheTag("directory"), cacheTag("gallery"), cacheTag(\photo:${id}`), cacheTag(`entry:${category}:${slug}`)`, etc.
• Backend: FrontendRevalidationService.revalidatePath() only sends {"path": "/..."} — no tag support

Proposed Changes

1. Add revalidateTag(tag: String) method to FrontendRevalidationService
2. Send {"tag": "gallery"} body format to /api/revalidate
3. Call revalidateTag("gallery") from gallery admin operations (promote hero, approve/reject media)
4. Call revalidateTag("directory") from directory admin operations

Files

• apps/api/src/main/kotlin/com/nosilha/core/shared/service/FrontendRevalidationService.kt
• Gallery and directory admin service classes that trigger revalidation

Context

This was identified during code review of PR #85 (Phase 1 performance optimization). The tag infrastructure was scaffolded on the frontend; this issue completes the backend integration (Phase 2).