perf: in-memory packet store — all reads from RAM, SQLite write-only

- PacketStore loads all packets into memory on startup (~11MB for 27K packets)
- Indexed by id, hash, observer, and node pubkey for fast lookups
- /api/packets, /api/packets/timestamps, /api/packets/:id all served from RAM
- MQTT ingest writes to both RAM + SQLite
- Configurable maxMemoryMB (default 1024MB) in config.json packetStore section
- groupByHash queries computed in-memory
- Packet store stats exposed in /api/perf
- Expected: /api/packets goes from 77ms to <1ms

config.example.json

@@ -43,5 +43,10 @@
     "nodeSearch": 10,
     "invalidationDebounce": 30,
     "_comment": "All values in seconds. Server uses these directly. Client fetches via /api/config/cache."
+  },
+  "packetStore": {
+    "maxMemoryMB": 1024,
+    "estimatedPacketBytes": 450,
+    "_comment": "In-memory packet store. maxMemoryMB caps RAM usage. All packets loaded on startup, served from RAM."
   }
 }