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>

docs/decisions/optimistic-concurrency.md

@@ -17,6 +17,7 @@ cross-editor ETag rotation). Refs #197.
 - [2026-07-12 (#269 — non-If-Match root writers force-write past a concurrent Version bump)](#2026-07-12-269--non-if-match-root-writers-force-write-past-a-concurrent-version-bump)
 - [2026-07-12 — Cross-editor ETag rotation completed for Collection/Playout config siblings (#269)](#2026-07-12--cross-editor-etag-rotation-completed-for-collectionplayout-config-siblings-269)
 - [2026-07-12 — If-Match evaluates per RFC 7232: valid-but-non-matching → 412, only grammar violations → 400 (#265)](#2026-07-12--if-match-evaluates-per-rfc-7232-valid-but-non-matching--412-only-grammar-violations--400-265)
+- [2026-07-18 — Concurrent same-item add is idempotent, not a 500: catch the unique-violation per provider (#308)](#2026-07-18--concurrent-same-item-add-is-idempotent-not-a-500-catch-the-unique-violation-per-provider-308)
 
 ---
 
@@ -254,3 +255,36 @@ worried about was already correct (each handler loads/validates → 404 before `
 first-party SPA only ever echoes the single canonical strong tag we emit, so no shipped client changes behavior;
 the change only makes a hand-written/tooling `If-Match` get the RFC-correct status. Docs: `api-conventions.md`
 §7a. Refs #265 #253 #197.
+
+## 2026-07-18 — Concurrent same-item add is idempotent, not a 500: catch the unique-violation per provider (#308)
+
+**Decision.** The `Add*ToCollection` family's membership pre-check (#269) is not atomic with the insert, so two
+*concurrent* adds of the same item both observe it absent and both stage the `CollectionItem` composite key; the
+loser's `SaveChangesForcingVersion` threw a unique/PK-violation `DbUpdateException` (SQLite error 19 / MySQL 1062)
+it did not catch → **500**. We now treat that loss as an **idempotent no-op**, not an error: the desired end state
+(the item is a member) already holds because the racing winner inserted it, rotated the ETag, and fanned out the
+rebuild.
+
+**Mechanism.** A `bool`-returning sibling `ConcurrencyExtensions.TrySaveChangesForcingVersion` wraps
+`SaveChangesForcingVersion` and catches *only* a classified unique/PK violation, returning `false`. The 10
+single-item handlers return `Unit.Default` on `false` (skip the reindex/rebuild fan-out — the winner did it). The
+bulk `AddItemsToCollection` handler cannot no-op — that would silently drop the non-colliding items when a batch
+partially overlaps a concurrent add — so it **retries** on a fresh context against recomputed membership (bounded
+loop; the common no-collision path runs once).
+
+**Provider seam.** Detection is provider-specific but the Application layer must not reference the provider
+packages, so it follows the existing `TvContext` static-provider-config idiom (`IsSqlite`, `LastInsertedRowId`): a
+settable `TvContext.IsUniqueConstraintViolation` delegate, pointed at `SqliteErrorClassifier` (extended codes 1555
+PK / 2067 UNIQUE) or `MySqlErrorClassifier` (`Number == 1062`) from `Startup.cs`, defaulting to a conservative
+"no" so an unwired provider never silently swallows a save failure. Chosen over DI to avoid threading a new
+service through 11 handlers, and because the provider discriminator already lives as a `TvContext` static.
+
+**Scope boundary.** `Add*ToPlaylist` is deliberately **untouched**: `PlaylistItem` has its own identity PK and no
+unique index on `(PlaylistId, MediaItemId)` — a playlist may legitimately contain the same item more than once, so
+there is no constraint to violate.
+
+**Tests.** A negative-control anchor proves the race genuinely throws a classified `DbUpdateException`; the fix's
+end-to-end handler tests reproduce a *real cross-connection* race via a shared-cache SQLite harness + a
+`SavingChanges` interceptor that inserts the conflicting row on another connection mid-save (the single-connection
+in-memory fixture cannot). Every fix-dependent test was verified to fail with the catch disabled. Mechanics:
+`api-conventions.md` §7a ("Idempotent insert under concurrency"). Refs #308 #269 #253.