refactor(db): collapse repository rollout scaffolding into single factory

Repositories are now the only data path. Replaces the 41 current-*-repository
wrapper files, the DATABASE_LAYER_REPOSITORY_ROLLOUT flag/alias map and the
unused database/runtime adapter with repositories/factory.ts, a plain
DatabaseContext type and an in-memory TestSqliteDatabase test harness.

src/backend/database/repositories/api-key-repository.ts

@@ -1,6 +1,6 @@
 import { eq, and } from "drizzle-orm";
 import { apiKeys, users } from "../db/schema.js";
-import type { DatabaseContext } from "../runtime/adapter.js";
+import type { DatabaseContext } from "./database-context.js";
 
 export type ApiKeyRecord = typeof apiKeys.$inferSelect;
 export type NewApiKeyRecord = typeof apiKeys.$inferInsert;