fix(sync): refresh a channel's details once they are a week old

Channel metadata (banner, avatar, description, counts) was fetched only the first time a
channel was seen: the subscription import asked only for channels with no details, and the
channel page only for ones missing a join date. 394 of 397 subscribed channels had July
metadata. Both now also refresh details older than DETAILS_MAX_AGE (7 days), through one
shared staleness rule; channels.list costs 1 unit per 50 channels.

backend/app/routes/channels.py

@@ -28,7 +28,7 @@ from app.models import (
 from app.routes._common import get_or_404, not_found, owned_or_404, youtube_refusal
 from app.sync.explore import explore_ingest_page
 from app.sync.runner import backfill_channel_now, run_recent_backfill
-from app.sync.subscriptions import apply_channel_details
+from app.sync.subscriptions import apply_channel_details, details_stale
 from app.utils import iso
 from app.youtube.client import YouTubeClient, YouTubeError
 from app.refusal import refused
@@ -297,10 +297,13 @@ def channel_detail(
     relationship to it (subscribed / explored) and how many of its videos are in the catalog.
     Lazily fills the About metadata (view count, join date, banner, links) the first time it's
     needed — a stub, or a channel last enriched before those fields existed (published_at is a
-    reliable sentinel: every real channel has a creation date). The enrich uses the API key
-    (1 unit, cached after), so it's skipped for the quota-less demo account."""
+    reliable sentinel: every real channel has a creation date) — and refreshes it once it is stale
+    (`DETAILS_MAX_AGE`), which is how a channel nobody subscribes to (an explored one, outside the
+    subscription import) gets a banner that still resolves. The enrich uses the API key (1 unit,
+    cached after), so it's skipped for the quota-less demo account."""
     channel = get_or_404(db, Channel, channel_id, what="channel")
-    if channel.published_at is None and not user.is_demo:
+    stale = channel.published_at is None or details_stale(channel, datetime.now(timezone.utc))
+    if stale and not user.is_demo:
         try:
             with (
                 quota.attribute(user.id, quota.QuotaAction.CHANNELS_DISCOVER),