feat(us-016): tracker route for HF models, endpoint dedup, purge logging

Tracker /v1/route now resolves HF model nodes (by hf_repo or short name)
in addition to preset models, using the same greedy interval-cover logic.
This allows distributed inference routing across two nodes each holding
half the model.

Endpoint dedup: re-registering the same endpoint atomically replaces the
old entry so stale registrations don't accumulate across node restarts.

Purge logging: tracker now prints when a node expires due to missed
heartbeats so operators can see dead nodes being removed.

Timing fix: heartbeat timeout raised from 30s to 90s (3 missed beats);
node heartbeat interval lowered from 30s to 20s to maintain margin.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/node/meshnet_node/startup.py

@@ -34,7 +34,7 @@ def _get_json(url: str, timeout: float = 10.0) -> dict:
         return json.loads(r.read())
 
 
-def _start_heartbeat(tracker_url: str, node_id: str, interval: float = 30.0) -> threading.Thread:
+def _start_heartbeat(tracker_url: str, node_id: str, interval: float = 20.0) -> threading.Thread:
     """Daemon thread that sends periodic heartbeats to the tracker."""
     def _loop() -> None:
         hb_url = f"{tracker_url}/v1/nodes/{node_id}/heartbeat"