refactor(connections): remove ConnectionConfig, migrate mapper to SQL Lab

Backend:
- Remove ConnectionConfig model, CRUD routes (connections.py), and tests
- Remove ensure_connection_configs_table from database.py
- Migrate MapperPlugin from psycopg2 direct PostgreSQL to SupersetSqlLabExecutor
- Add DatasetMapper.get_sqllab_mappings() with default information_schema query
- Update MapColumnsRequest: connection_id → database_id + sql_query
- Fix pydantic_settings v2 deprecation (Field(env=...) → validation_alias)
- Clean up app.py, routes/__init__.py, database.py imports

Frontend:
- Remove DatabaseConnectionsTab, ConnectionForm, ConnectionList components
- Remove /settings/connections route page and Navbar link
- Remove connectionService.js, connections i18n files
- Update MapperTool.svelte: postgres → sqllab source
- Update Map Columns modal: database selector + SQL query instead of connection_id
- Clean up settings page tabs, nav links, test mocks
- Clean up i18n keys (settings, nav, mapper)

Tests: 25/25 pass (14 030-feature + 11 route tests)
Build: succeeds

backend/src/core/database.py

@@ -4,7 +4,6 @@
 # @LAYER: Infrastructure
 # @RELATION DEPENDS_ON -> [MappingModels]
 # @RELATION DEPENDS_ON -> [auth_config]
-# @RELATION DEPENDS_ON -> [ConnectionConfig]
 #
 # @INVARIANT: A single engine instance is used for the entire application.
 
@@ -20,13 +19,11 @@ from ..models import (
     auth as _auth_models,  # noqa: F401
     clean_release as _clean_release_models,  # noqa: F401
     config as _config_models,  # noqa: F401
-    connection as _connection_models,  # noqa: F401
     dataset_review as _dataset_review_models,  # noqa: F401
     llm as _llm_models,  # noqa: F401
     profile as _profile_models,  # noqa: F401
     task as _task_models,  # noqa: F401
 )
-from ..models.connection import ConnectionConfig
 from ..models.mapping import Base
 from .auth.config import auth_config
 from .logger import belief_scope, logger
@@ -367,26 +364,6 @@ def _ensure_auth_users_columns(bind_engine):
 # #endregion _ensure_auth_users_columns
 
 
-# #region ensure_connection_configs_table [C:3] [TYPE Function]
-# @BRIEF Ensures the external connection registry table exists in the main database.
-# @PRE: bind_engine points to the application database.
-# @POST: connection_configs table exists without dropping existing data.
-# @RELATION DEPENDS_ON -> [ConnectionConfig]
-def ensure_connection_configs_table(bind_engine):
-    with belief_scope("ensure_connection_configs_table"):
-        try:
-            ConnectionConfig.__table__.create(bind=bind_engine, checkfirst=True)
-        except Exception as migration_error:
-            logger.explore(
-                "ConnectionConfig table ensure failed",
-                extra={"src": "database", "error": str(migration_error)},
-            )
-            raise
-
-
-# #endregion ensure_connection_configs_table
-
-
 # #region _ensure_filter_source_enum_values [C:3] [TYPE Function]
 # @BRIEF Adds missing FilterSource enum values to the PostgreSQL native filtersource type.
 # @PRE: bind_engine points to application database with imported_filters table.
@@ -686,7 +663,6 @@ def _ensure_dictionary_entries_columns(bind_engine):
 # @PRE: engine, tasks_engine and auth_engine are initialized.
 # @POST: Database tables created in all databases.
 # @SIDE_EFFECT: Creates physical database files if they don't exist.
-# @RELATION CALLS -> [ensure_connection_configs_table]
 # @RELATION CALLS -> [_ensure_filter_source_enum_values]
 # @RELATION CALLS -> [_ensure_dataset_review_session_columns]
 # @RELATION CALLS -> [_ensure_dictionary_entries_columns]
@@ -700,7 +676,6 @@ def init_db():
         _ensure_user_dashboard_preferences_health_columns(engine)
         _ensure_git_server_configs_columns(engine)
         _ensure_auth_users_columns(auth_engine)
-        ensure_connection_configs_table(engine)
         _ensure_filter_source_enum_values(engine)
         _ensure_dataset_review_session_columns(engine)
         _ensure_translation_jobs_columns(engine)