feat(us-025): model usage statistics with rolling RPM windows and SQLite persistence

Adds _RollingCounter and _StatsCollector to the tracker: three rolling windows
(hour=60×1min, day=24×1hr, month=30×1day) track request RPMs per model.
GET /v1/stats returns combined local + peer stats. POST /v1/stats/gossip lets
trackers push their local slice for additive merge in hive mode.
SQLite persistence via --stats-db flag; stats survive tracker restarts.
Records a stat for each proxied /v1/chat/completions request.

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

packages/tracker/meshnet_tracker/cli.py

@@ -33,6 +33,12 @@ def main() -> None:
         default=None,
         help="This tracker's own URL as seen by peers (auto-derived from --host/--port if omitted)",
     )
+    start_cmd.add_argument(
+        "--stats-db",
+        default=None,
+        metavar="PATH",
+        help="SQLite database path for persistent model usage statistics",
+    )
 
     args = parser.parse_args()
 
@@ -44,6 +50,7 @@ def main() -> None:
             heartbeat_timeout=args.heartbeat_timeout,
             cluster_peers=cluster_peers or None,
             cluster_self_url=args.self_url,
+            stats_db=getattr(args, "stats_db", None),
         )
         port = server.start()
         print(f"meshnet-tracker listening on http://{args.host}:{port}", flush=True)