feat(db): FleetDecky table mirrors decnet-state.json into the DB

Adds a fleet_deckies table so DB-only consumers (orchestrator, web
dashboard, REST API) can see unihost / MACVLAN / IPVLAN deckies
without reading the JSON state file. Mirrors DeckyShard field-for-field.

Composite PK (host_uuid, name) future-proofs for a mothership that
runs both a local fleet and acts as a swarm master. host_uuid defaults
to the "local" sentinel — no FK to swarm_hosts because the local
mothership isn't enrolled as a worker.

Repo additions: upsert_fleet_decky, delete_fleet_decky,
list_fleet_deckies, list_running_fleet_deckies,
update_fleet_decky_state, plus list_running_deckies which unions
topology + fleet + shard sources for the orchestrator.

Smoke-tested round-trip against MySQL: upsert, list_running, union
view (source="fleet"), delete.

decnet/web/db/models/__init__.py

@@ -49,6 +49,10 @@ from .deploy import (
     MutateIntervalRequest,
     PurgeResponse,
 )
+from .fleet import (
+    LOCAL_HOST_SENTINEL,
+    FleetDecky,
+)
 from .health import (
     ComponentHealth,
     HealthResponse,
@@ -182,6 +186,9 @@ __all__ = [
     "DeployResponse",
     "MutateIntervalRequest",
     "PurgeResponse",
+    # fleet
+    "LOCAL_HOST_SENTINEL",
+    "FleetDecky",
     # health
     "ComponentHealth",
     "HealthResponse",