fix(jellyfin): item-aware local artwork mirroring with undo reconciliation

[SaxxyToo]

What this fixes

With LOCAL_ARTWORK enabled against Jellyfin, mirrored local files landed in the wrong place or could escape the configured media mapping, and undo left them behind — so Jellyfin's local image provider would re-import stale art on its next refresh, silently reverting an undo.

Item-aware destinations

The Jellyfin/Emby adapter now reports an item's path and item type (movie/show/season/episode) via getItemMediaLocation, matched on the exact item id; virtual/remote/unknown types return null. The mirror uses the type to place files where Jellyfin actually looks:

• Show / season — folder.* inside the item's own directory (previously landed in the library root or the show folder).
• Episode — basename-thumb.* (previously clobbered the season folder's art).
• Movie — folder.* in dedicated single-video folders; basename-poster.* / basename-fanart.* in shared multi-video folders, so applying one movie no longer overwrites a sibling's poster.
• Season overrides — season mirrors skip when the parent show directory carries season-prefixed override images, matching Jellyfin's own provider precedence.

Containment and safe writes

• Realpath-checked containment: .. traversal and symlink escapes are refused. Nothing is read, backed up, or moved outside the mapped MEDIA_PATH_MAP root. (Reproduced the escape before the fix.)
• Conflicting local images (poster.jpg, cover.jpg, …) are reconciled — backed up and removed — even when the canonical file already matches, closing the stale-ghost path.
• Writes stage to a temp file, re-check for races, then atomically rename; per-folder serialization; cleanup on failure; backups use exclusive creates.

Undo reconciliation

Undo now mirrors the restored server artwork back to the local file, compare-and-set against the pre-undo bytes:

• Refuses when the local file changed independently — never overwrites art you edited outside PosterPilot.
• Backs up before removal; removals only touch matching conflicts; unrelated images are untouched.
• Best-effort: a mirror failure is logged and never fails the server-side undo. Wired into the real executor through createLocalArtworkWriterFromEnv.

Verification

• bun run check — 0 errors; bun run lint — clean; bun run build — success.
• bun run test — 2,533 passed, 2 skipped.
• Local-artwork suite 10 → 43 tests; 11 new undo-mirror tests; typed-location forwarding tests for apply and custom upload; new adapter location test file.
• Component tests fail identically before and after (vitest browser connection drops mid-run) — pre-existing, environment-level.

Follow-ups (not in this change)

• Undo still unconditionally unlocks the metadata field on Jellyfin, where the lock is item-wide rather than per-image, so undoing one slot drops protection from the others.
• vitest.config.ts still uses the removed-in-v4 poolOptions (warning only).