feat: fleet-wide MACVLAN sniffer microservice

Replace per-decky sniffer containers with a single host-side sniffer
that monitors all traffic on the MACVLAN interface. Runs as a background
task in the FastAPI lifespan alongside the collector, fully fault-isolated
so failures never crash the API.

- Add fleet_singleton flag to BaseService; sniffer marked as singleton
- Composer skips fleet_singleton services in compose generation
- Fleet builder excludes singletons from random service assignment
- Extract TLS fingerprinting engine from templates/sniffer/server.py
  into decnet/sniffer/ package (parameterized for fleet-wide use)
- Sniffer worker maps packets to deckies via IP→name state mapping
- Original templates/sniffer/server.py preserved for future use

decnet/services/sniffer.py

@@ -16,6 +16,7 @@ class SnifferService(BaseService):
     name = "sniffer"
     ports: list[int] = []
     default_image = "build"
+    fleet_singleton = True
 
     def compose_fragment(
         self,