feat(api): per-service config schema endpoint + PUT/POST update+apply for fleet & topology

- GET /topologies/services/{name}/schema serves the declared ServiceConfigField
  metadata so the Inspector can auto-render forms.
- PUT  /(topologies/{id}/)deckies/{decky}/services/{svc}/config persists the
  validated dict (DB + compose); container untouched (Save).
- POST /(topologies/{id}/)deckies/{decky}/services/{svc}/apply persists then
  force-recreates <decky>-<svc> so the new env takes effect (Apply, destructive).
- New engine helper update_service_config wires both fleet and topology paths
  through the existing _persist_fleet_change / _rerender_topology_compose
  machinery; emits decky.<name>.service_config_changed on the bus.

decnet/web/db/models/__init__.py

@@ -67,7 +67,11 @@ from .decky import (
     DeckyFileDeleteRequest,
     DeckyFileDropRequest,
     DeckyServiceAddRequest,
+    DeckyServiceConfigRequest,
+    DeckyServiceConfigResponse,
     DeckyServicesResponse,
+    ServiceConfigFieldDTO,
+    ServiceSchemaResponse,
 )
 from .fleet import (
     LOCAL_HOST_SENTINEL,
@@ -231,8 +235,12 @@ __all__ = [
     "DeckyFileDeleteRequest",
     "DeckyFileDropRequest",
     "DeckyServiceAddRequest",
+    "DeckyServiceConfigRequest",
+    "DeckyServiceConfigResponse",
     "DeckyServicesResponse",
     "FleetDecky",
+    "ServiceConfigFieldDTO",
+    "ServiceSchemaResponse",
     # health
     "ComponentHealth",
     "HealthResponse",