Weather review fixes: zone-fetch time budget, prod NWS_USER_AGENT (Refs #237)

- Cap one refresh's NWS zone fetches at 15 s wall clock. They run under the
  feed lock, so a hung zone endpoint could otherwise stall every
  /weather/alerts request for minutes; unfinished zones retry next refresh.
- Prod c2-core.env.j2 never set NWS_USER_AGENT, so the VM would have sent the
  contact-less placeholder. Default it to the site URL (overridable via
  vault_nws_user_agent); warn on the .env.example example.com placeholder too.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

drb-c2-core/app/internal/weather.py

@@ -48,6 +48,11 @@ ZONE_CONCURRENCY = 6
 # the next refresh (the zone cache fills across refreshes); meanwhile the alert
 # shows in zone_only or with the zones resolved so far.
 MAX_ZONE_FETCHES_PER_REFRESH = 80
+# Wall-clock cap on one refresh's zone fetches. The refresh runs under the feed
+# lock, so without it a slow zone endpoint (20 s timeout x 80 fetches / 6 wide)
+# could hold every /weather/alerts request for minutes. Unfinished zones are
+# simply not cached and are retried next refresh.
+ZONE_BUDGET_S = 15.0
 
 WIND_TTL_S = 5 * 60
 WIND_CELL_DEG = 0.5      # wind cache cell; one upstream call per cell per TTL
@@ -61,7 +66,7 @@ _warned_ua = False
 def _headers(accept: str) -> Dict[str, str]:
     global _warned_ua
     ua = settings.nws_user_agent or DEFAULT_USER_AGENT
-    if ua == DEFAULT_USER_AGENT and not _warned_ua:
+    if (ua == DEFAULT_USER_AGENT or "example.com" in ua) and not _warned_ua:
         _warned_ua = True
         logger.warning("NWS_USER_AGENT is the placeholder -- set it to 'app (contact email)'; NWS may block us")
     return {"User-Agent": ua, "Accept": accept}
@@ -193,7 +198,12 @@ async def _resolve_zones(client: "httpx.AsyncClient", alerts: List[Dict[str, Any
                     wanted.append(z)
     if wanted:
         sem = asyncio.Semaphore(ZONE_CONCURRENCY)
-        await asyncio.gather(*(_fetch_zone(client, z, sem) for z in wanted[:MAX_ZONE_FETCHES_PER_REFRESH]))
+        try:
+            await asyncio.wait_for(
+                asyncio.gather(*(_fetch_zone(client, z, sem) for z in wanted[:MAX_ZONE_FETCHES_PER_REFRESH])),
+                timeout=ZONE_BUDGET_S)
+        except asyncio.TimeoutError:
+            logger.warning(f"NWS zone fetches over {ZONE_BUDGET_S:.0f}s budget; the rest resolve next refresh")
     for a in alerts:
         if a["geometry"] is not None:
             continue