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>

drizzle.test.config.ts

@@ -0,0 +1,11 @@
+import { defineConfig } from 'drizzle-kit';
+
+if (!process.env.TEST_DATABASE_URL) throw new Error('TEST_DATABASE_URL is not set');
+
+export default defineConfig({
+	schema: './src/lib/server/db/schema.ts',
+	dialect: 'sqlite',
+	dbCredentials: { url: process.env.TEST_DATABASE_URL },
+	verbose: true,
+	strict: true
+});