Fix four defects flagged by S19/S20/S24; tighten scenarios

1. tagService.AssignTag and RemoveTag now use verifyModifyAccess
   (owner role required) instead of verifyAccess. Tags are global
   state visible to every user with access to a media item, so a
   viewer must not be able to add or remove them. Favorites and
   notes stay on verifyAccess because they're per-user data
   (favorites.user_id, media_notes.user_id) and don't affect anyone
   else. Verified via curl: viewer POST /media/{id}/tags now 403,
   admin still 200.

2. serveFileResult now emits a strong ETag header
   ("<size>-<mtime-nanos>") before calling http.ServeContent. Go's
   ServeContent honours If-None-Match when ETag is set, so iOS
   audio clients and podcast apps can revalidate cached downloads
   with conditional GETs. Verified via curl: ETag present on
   /stream; If-None-Match matching the ETag returns 304.

3. MediaFilter gains IncludeDeleted flag; ListMedia skips the
   implicit `deleted_at IS NULL` predicate when it is set.
   FSScanner.loadExistingMedia now passes IncludeDeleted=true so
   the dedup map includes soft-deleted rows. Previously a re-scan
   of a soft-deleted file tried to CreateMedia and hit the
   UNIQUE(set_id, rel_path) constraint, failing the whole scan and
   setting progress.last_error. Now the rescan skips the row
   cleanly; soft-delete sticks.

4. FSScanner.reconcileOrphans soft-deletes media rows whose
   underlying file disappeared between scans. The scanner used to
   only walk files that exist and never compare against the DB,
   leaving phantom rows in GET /api/v1/media that 404'd on stream.
   Verified via curl: rm /testdata/.../orphan.mp3, rescan, row
   now has deleted_at != NULL.

Scenarios updated to lock in the fixed behaviour:
  S19 step 15 — viewer tag-add now asserts 403, not 200.
  S20 step 14 — asserts ETag is present and If-None-Match → 304.
  S24 step 13 — asserts clean rescan (no last_error from UNIQUE).
  S24 step 20 — asserts orphan rows are soft-deleted by rescan.

Verified: full Go unit suite passes; 25/25 LLM e2e scenarios;
22/22 Playwright e2e-web.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

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