feat: surface fleet_singleton flag on /topologies/services

Adds a fleet_singletons array to ServiceCatalogResponse so per-decky
add UIs can filter out services like LLMNR that run once fleet-wide
(and would 422 server-side at the live add endpoint).

The existing 'services: list[str]' field is unchanged for back-compat
with MazeNET/useMazeApi.ts:257; the new field is additive.

decnet_web/src/hooks/useServiceRegistry.ts wraps the endpoint with a
module-scoped cache (registry only changes on BYOS install / plugin
drop, neither of which happens mid-session) and exposes a precomputed
.perDecky list so consumers don't need to re-derive the diff.

decnet/web/db/models/topology.py

@@ -404,6 +404,12 @@ class NotEditableResponse(BaseModel):
 
 class ServiceCatalogResponse(BaseModel):
     services: list[str]
+    # Subset of ``services`` that run once fleet-wide (LLMNR, etc.) and
+    # therefore can't be added to a single decky.  Per-decky add UIs
+    # filter these out so the operator never picks an option that the
+    # server would reject as 422.  Empty when the registry has no
+    # singletons.
+    fleet_singletons: list[str] = PydanticField(default_factory=list)
 
 
 class ArchetypeEntry(BaseModel):

decnet/web/router/topology/api_catalog.py

@@ -42,7 +42,14 @@ router = APIRouter()
 async def api_list_services(
     _viewer: dict = Depends(require_viewer),
 ) -> ServiceCatalogResponse:
-    return ServiceCatalogResponse(services=all_service_names())
+    from decnet.services.registry import all_services
+    registry = all_services()
+    return ServiceCatalogResponse(
+        services=all_service_names(),
+        fleet_singletons=[
+            name for name, svc in registry.items() if svc.fleet_singleton
+        ],
+    )
 
 
 @router.get(