fix(core): race-safe matching, visibility-checked ID matches, child-preserving merges

- migration 0002: unique index on works (parent_id, position)
- matchWork retries the whole transaction on a unique violation (3 attempts)
- addMissingExternalIds uses ON CONFLICT DO NOTHING
- ID matches only return shared works or the user's own; private works cannot carry external ids
- adminMergeWorks requires shared works and repoints non-colliding children
- draft is computed for the aliased work id

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/db/src/migrations/0002_works_position_unique.ts

@@ -0,0 +1,16 @@
+import type { Kysely } from "kysely";
+import type { Migration } from "kysely/migration";
+
+/**
+ * One child per (parent, position), so concurrent matches cannot duplicate seasons or episodes.
+ * NULLs are distinct on both dialects: top-level works and children without a position are unaffected.
+ */
+export const worksPositionUnique: Migration = {
+  async up(db: Kysely<any>) {
+    await db.schema.createIndex("works_parent_id_position_uq").unique().on("works").columns(["parent_id", "position"]).execute();
+  },
+
+  async down(db: Kysely<any>) {
+    await db.schema.dropIndex("works_parent_id_position_uq").execute();
+  },
+};

packages/db/src/migrations/index.ts

@@ -1,7 +1,9 @@
 import type { Migration } from "kysely/migration";
 import { initial } from "./0001_initial.ts";
+import { worksPositionUnique } from "./0002_works_position_unique.ts";
 
 /** All migrations, keyed by sortable name. Imported statically so bundles need no filesystem. */
 export const migrations: Record<string, Migration> = {
   "0001_initial": initial,
+  "0002_works_position_unique": worksPositionUnique,
 };