fix(308): idempotent concurrent Add*ToCollection instead of a composite-PK 500

Two concurrent adds of the same item both membership-check it absent, both
insert the CollectionItem composite key, and the loser's
SaveChangesForcingVersion threw an uncaught DbUpdateException (SQLite 19 /
MySQL 1062) -> 500. Now the loser is an idempotent no-op.

- ConcurrencyExtensions.TrySaveChangesForcingVersion: bool-returning sibling
  that catches only a classified unique/PK violation and returns false.
- 10 single-item Add*ToCollection handlers: return Unit.Default (no-op, skip
  fan-out) on false — the racing winner already inserted + rotated + rebuilt.
- Bulk AddItemsToCollection: retry on a fresh context against recomputed
  membership so a partial-overlap collision doesn't drop the non-colliding
  items (bounded loop; common no-collision path runs once).
- Provider detection via a TvContext.IsUniqueConstraintViolation static
  delegate (matches the existing IsSqlite/LastInsertedRowId provider seam),
  wired from Startup to SqliteErrorClassifier / MySqlErrorClassifier.
- Add*ToPlaylist is NOT affected (PlaylistItem has its own identity PK; a
  playlist may legitimately contain the same item more than once).

Tests: a negative-control anchor proves the race genuinely throws a classified
exception; end-to-end handler tests reproduce a real cross-connection race via
a shared-cache SQLite harness + a SavingChanges interceptor (the single-conn
in-memory fixture cannot). Every fix-dependent test verified to fail with the
catch disabled.

Docs: api-conventions.md §7a (idempotent insert under concurrency) +
decisions/optimistic-concurrency.md.

fixes #308

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ErsatzTV.Application/MediaCollections/Commands/AddImageToCollectionHandler.cs

@@ -1,4 +1,4 @@
-using System.Threading.Channels;
+using System.Threading.Channels;
 using ErsatzTV.Application.Playouts;
 using ErsatzTV.Application.Search;
 using ErsatzTV.Core;
@@ -54,7 +54,13 @@ public class AddImageToCollectionHandler : IRequestHandler<AddImageToCollection,
         // force-write past a concurrent replace-all Version bump — this add takes no If-Match, so a
         // benign race must not 500 (#253/#269 §7a). Post-commit enqueues run on CancellationToken.None.
         parameters.Collection.Version++;
-        await dbContext.SaveChangesForcingVersion(CancellationToken.None);
+        if (!await dbContext.TrySaveChangesForcingVersion(CancellationToken.None))
+        {
+            // A concurrent add of this same item won the composite-PK race and already inserted the row,
+            // rotated the collection ETag, and fanned out the rebuild — so this is now an idempotent
+            // no-op. Skip our reindex/rebuild fan-out (the winner already did it). #308
+            return Unit.Default;
+        }
 
         await _searchChannel.WriteAsync(new ReindexMediaItems([parameters.Image.Id]), CancellationToken.None);