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/src/scripts/seed_permissions.py

@@ -51,6 +51,8 @@ INITIAL_PERMISSIONS = [
     {"resource": "plugin:storage", "action": "WRITE"},
     {"resource": "plugin:debug", "action": "EXECUTE"},
     {"resource": "git_config", "action": "READ"},
+    # Connection Settings Permissions
+    {"resource": "settings.connections", "action": "MANAGE"},
     # Dataset Review Permissions
     {"resource": "dataset:session", "action": "READ"},
     {"resource": "dataset:session", "action": "MANAGE"},