fix(496): harden the legacy path sweep against MySQL collation (re-review Low)

Re-review returned MERGEABLE with one Low, MySQL-collation-dependent edge in
FlagFileNotFoundByPaths: the C#-side Except diff is ordinal, but `MF.Path IN @LocalPaths`
runs under MySQL's case-insensitive default collation, so a still-reported identified row
differing only in case from an absent legacy row could be matched and flagged missing.

Match on the indexed PathHash instead of collated Path text, and re-state the
NOT EXISTS (JellyfinMusicVideo) guard so the identity pass and the legacy pass are disjoint
by construction rather than by the caller's diff being correct. SQLite was unaffected.

docs/decisions.md

@@ -3857,7 +3857,7 @@ This is a **second, independent** refusal on the same guard, plus a decision not
 ## 2026-07-25 — Music videos carry a per-library server identity; reconciliation is an itemId diff + soft trash (#496)
 `key: scan.musicvideo-server-identity` · `status: active` · `since: 2026-07-25` · `supersedes: scan.musicvideo-reconciliation@2026-07-20` · `superseded-by: none`
 **Rule:** Jellyfin music videos carry a per-library server identity (`JellyfinMusicVideo : MusicVideo` with `ItemId`/`Etag`, TPT table + ItemId index), so `JellyfinMusicVideoLibraryScanner` folds onto a shared `MediaServerMusicVideoLibraryScanner` base that diffs the **server item id** and soft-trashes (`FlagFileNotFound`) instead of diffing local paths and hard-deleting. Rows predating the identity are **adopted in place** — the identity row is inserted against the same `MediaItem` id, scoped to the scanned library's own `LibraryPath` — never deleted and re-added.
-**Signals:** music-video server identity, JellyfinMusicVideo ItemId/Etag, itemId diff, soft FileNotFound trash, adoption of pre-identity rows, cross-library false-trash, path-replaced PathHash · paths: `JellyfinMusicVideo`, `JellyfinMusicVideoRepository`, `IMediaServerMusicVideoRepository`, `MediaServerMusicVideoLibraryScanner`, `JellyfinMusicVideoLibraryScanner` · issues: #496, #494, #477, #488, #497, #500
+**Signals:** music-video server identity, JellyfinMusicVideo ItemId/Etag, itemId diff, soft FileNotFound trash, adoption of pre-identity rows, cross-library false-trash, path-replaced PathHash · paths: `JellyfinMusicVideo`, `JellyfinMusicVideoRepository`, `IMediaServerMusicVideoRepository`, `MediaServerMusicVideoLibraryScanner`, `JellyfinMusicVideoLibraryScanner` · issues: #496, #494, #477, #488, #497, #500, #606
 **Mechanics:** dual-provider migration `Add_JellyfinMusicVideo`; `GetOrAdd` takes `localPath` explicitly and never reads the projection's path; adoption probe joins `MediaFile.PathHash` + `NOT EXISTS (JellyfinMusicVideo)` filtered to the library's `LibraryPath`; `GetByItemId` filters `LibraryPath.LibraryId`; legacy (identity-less) rows reconciled by `GetExistingLegacyMusicVideoPaths` + `FlagFileNotFoundByPaths` and counted into the #477 guard; `ScanLibrary_Should_Adopt_Using_The_PathReplaced_Local_Path`, `ScanLibrary_Should_Adopt_PreExisting_MusicVideo_Preserving_Identity_And_Collections`, `ScanLibrary_Should_Not_Adopt_A_MusicVideo_Owned_By_Another_LibraryPath`, `ScanLibrary_Should_Not_Resolve_An_ItemId_Owned_By_Another_Library`, `ScanLibrary_Should_Flag_A_Legacy_Row_The_Server_No_Longer_Reports`
 
 #494 gave music videos a trash sweep but had to key it on `(LibraryPathId, path)` and hard-delete, because
@@ -3904,4 +3904,5 @@ owner's sweep **destroyed** the row the other library still served. This is the
   (`MediaFileAlreadyExists` is a global path-hash guard), so a second library serving the same file still gets
   no row of its own — exactly as for movies/TV. What changes is that the first library's sweep now *flags*
   rather than *destroys* that shared row. The unrecoverable data loss is gone; the shared-row limitation is a
-  whole-app property, not a music-video one.
+  whole-app property, not a music-video one, and is tracked separately as **#606**. #496's Done-when was
+  amended to this parity wording rather than ticked as literally written.