fix(discovery): cache a successful but empty Tailscale lookup

The host cache was gated on the list being non-empty, so "queried fine, no
eligible peers" looked exactly like a cold cache and every caller paid for
another `tailscale status --json` — a subprocess with a 5s timeout.

Gate on the timestamp instead. Failures still leave the timestamp unset, so a
missing binary, a non-zero exit or unparseable output stays retryable rather
than being cached for the full TTL.

Ported from public `dev` (`affaee1e`, #6228 upstream), with its test.

src/model_discovery.py

@@ -38,7 +38,10 @@ def discover_tailscale_hosts() -> List[str]:
     global _hosts_cache, _hosts_cache_time
 
     now = time.time()
-    if _hosts_cache and (now - _hosts_cache_time) < _HOSTS_CACHE_TTL:
+    # Gate on the timestamp, not the list: a successful query that found no
+    # eligible peers is a real answer, and testing the list's truthiness made
+    # that case re-run `tailscale status` (up to a 5s timeout) on every call.
+    if _hosts_cache_time and (now - _hosts_cache_time) < _HOSTS_CACHE_TTL:
         return list(_hosts_cache)
 
     hosts = []