feat(core): implement ConnectionService and DbExecutor for direct DB insert (US11-US12)

- ConnectionService: CRUD for DatabaseConnection in GlobalSettings with
  EncryptionManager-backed password encryption/decryption, test connectivity
- DbExecutor: asyncpg (PostgreSQL) and clickhouse-connect (ClickHouse) drivers
  with per-connection connection pooling
- config_models.py: promote GlobalSettings.connections from list[dict] stub
  to list[DatabaseConnection] with full Pydantic model validation
- settings.py: +5 CRUD endpoints for connections (create, read, update, delete, test)
- requirements.txt: add asyncpg>=0.29.0, clickhouse-connect>=0.7.0
- seed_permissions.py: register settings.connections.manage permission

backend/requirements.txt

@@ -57,6 +57,10 @@ playwright
 tenacity
 Pillow
 ruff>=0.11.0
+# Direct database drivers for DbExecutor (optional)
+asyncpg>=0.29.0
+clickhouse-connect>=0.7.0
+pymysql>=1.1.0
 sqlparse>=0.5.0
  lingua-language-detector==2.1.1
 testcontainers[postgres]>=4.0