Add test infrastructure and signin migration tests

- Add test-specific Drizzle config and database connection
- Create test version of auth module using test database
- Add comprehensive integration tests for signin migration logic
- Add unit tests for deduplication algorithm
- Tests cover edge cases like multiple duplicates, timing, and error handling

🤖 Generated with [Claude Code](https://claude.ai/code)

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

src/lib/server/db/test.ts

@@ -0,0 +1,9 @@
+import { drizzle } from 'drizzle-orm/bun-sqlite';
+import { Database } from 'bun:sqlite';
+import * as schema from './schema';
+
+if (!Bun.env.TEST_DATABASE_URL) throw new Error('TEST_DATABASE_URL is not set');
+
+const testClient = new Database(Bun.env.TEST_DATABASE_URL);
+
+export const testDb = drizzle(testClient, { schema });