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/app.py

@@ -28,7 +28,6 @@ from .api.routes import (
     assistant,
     clean_release,
     clean_release_v2,
-    connections,
     dashboards,
     dataset_review,
     datasets,
@@ -215,7 +214,6 @@ app.add_middleware(TraceContextMiddleware)
 # @RELATION DEPENDS_ON -> [PluginsRouter]
 # @RELATION DEPENDS_ON -> [TasksRouter]
 # @RELATION DEPENDS_ON -> [SettingsRouter]
-# @RELATION DEPENDS_ON -> [ConnectionsRouter]
 # @RELATION DEPENDS_ON -> [ReportsRouter]
 # @RELATION DEPENDS_ON -> [LlmRoutes]
 # @RELATION DEPENDS_ON -> [CleanReleaseV2Api]
@@ -225,9 +223,6 @@ app.include_router(admin.router)
 app.include_router(plugins.router, prefix="/api/plugins", tags=["Plugins"])
 app.include_router(tasks.router, prefix="/api/tasks", tags=["Tasks"])
 app.include_router(settings.router, prefix="/api/settings", tags=["Settings"])
-app.include_router(
-    connections.router, prefix="/api/settings/connections", tags=["Connections"]
-)
 app.include_router(environments.router, tags=["Environments"])
 app.include_router(mappings.router, prefix="/api/mappings", tags=["Mappings"])
 app.include_router(migration.router)