S4 review: voice-check checked_at is the oldest snapshot, not now (server-26#225)

The picker footer reads "Permissions checked N ago" from checked_at, but
voice_check stamped it with the response time. A denied result is cached
for 10 min, so a user who fixed the permission in Discord and reopened the
picker saw "checked 2s ago · Needs fix" from a stale snapshot. checked_at
now carries the oldest fetched_at behind the answer (now when there is none).

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

drb-c2-core/app/routers/discord_checks.py

@@ -87,6 +87,16 @@ def _channel_state(rows: list[dict], enabled: bool) -> str:
     return "no_bot"
 
 
+def _checked_at(snaps: dict) -> str:
+    """When Discord was last actually asked: the OLDEST cached snapshot behind
+    this answer (a denied result is cached for 10 min), so the picker's
+    "Permissions checked N ago" never calls a stale answer fresh (server-26#225
+    S4). No snapshot (check off, no member bots) = now."""
+    times = [s.fetched_at for s in snaps.values() if s is not None and getattr(s, "fetched_at", None)]
+    ts = min(times) if times else time.time()
+    return datetime.fromtimestamp(ts, timezone.utc).isoformat()
+
+
 async def voice_check(*, guild_id: str, org_id: Optional[str], node_id: Optional[str],
                       channels: list[dict], channel_id: Optional[str], refresh: bool,
                       with_token_ids: bool) -> dict:
@@ -171,7 +181,7 @@ async def voice_check(*, guild_id: str, org_id: Optional[str], node_id: Optional
         "guild_id": guild_id,
         "node_id": node_id,
         "enabled": enabled,
-        "checked_at": datetime.now(timezone.utc).isoformat(),
+        "checked_at": _checked_at(snaps),
         "refreshed": refresh,
         "channels": out_channels,
         "invite": invite,