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/sync/subscriptions.py

@@ -1,8 +1,8 @@
 """Import the authenticated user's YouTube subscriptions and channel metadata."""
 
-from datetime import datetime, timezone
+from datetime import datetime, timedelta, timezone
 
-from sqlalchemy import select
+from sqlalchemy import or_, select
 from sqlalchemy.orm import Session
 
 from app import logs
@@ -47,6 +47,27 @@ def _external_links(branding: dict) -> list | None:
     return out or None
 
 
+# How old a channel's metadata may get before it is fetched again. Banners, avatars, descriptions and
+# counts change on YouTube, and an old banner URL stops resolving at all — without a refresh a channel
+# kept whatever it had the day it was first seen. A week keeps counts roughly current at almost no
+# cost: `channels.list` is 1 quota unit per 50 channels, and only the stale ones are asked for.
+DETAILS_MAX_AGE = timedelta(days=7)
+
+
+def details_stale(channel: Channel, now: datetime) -> bool:
+    """Whether `channel`'s metadata is missing or older than DETAILS_MAX_AGE (see `details_stale_clause`)."""
+    synced = channel.details_synced_at
+    return synced is None or synced < now - DETAILS_MAX_AGE
+
+
+def details_stale_clause(now: datetime):
+    """`details_stale` as a WHERE clause, so the batch import and a single channel agree on "stale"."""
+    return or_(
+        Channel.details_synced_at.is_(None),
+        Channel.details_synced_at < now - DETAILS_MAX_AGE,
+    )
+
+
 def apply_channel_details(db: Session, items: list[dict]) -> None:
     now = datetime.now(timezone.utc)
     for item in items:
@@ -92,7 +113,7 @@ def should_prune(fetched_count: int, existing_count: int) -> bool:
 def import_subscriptions(db: Session, user: User) -> dict:
     """Pull all subscriptions for `user`, upsert channels + subscription links, prune
     channels the user has unsubscribed from on YouTube, and fetch channel details for
-    any channel we don't have details for yet."""
+    any channel whose details are missing or stale (`DETAILS_MAX_AGE`)."""
     fetched: dict[str, dict] = {}
     new_channels = 0
 
@@ -145,12 +166,12 @@ def import_subscriptions(db: Session, user: User) -> dict:
                     removed += 1
         db.commit()
 
-        # Fetch details for channels that don't have them yet.
+        # Fetch details for channels that don't have them yet, or had them too long ago.
         need_details = (
             db.execute(
                 select(Channel).where(
                     Channel.id.in_(fetched.keys()),
-                    Channel.details_synced_at.is_(None),
+                    details_stale_clause(datetime.now(timezone.utc)),
                 )
             )
             .scalars()