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/tests/test_channel_details_refresh.py

@@ -0,0 +1,107 @@
+"""A channel's metadata is fetched again once it is stale, not only the first time it is seen.
+
+Banners, avatars, descriptions and counts change on YouTube, and an old banner URL stops resolving:
+a channel that kept the metadata of the day it was first imported showed a dead banner forever. The
+subscription import and the channel page both refresh what is older than `DETAILS_MAX_AGE`, and
+leave fresh metadata alone (each refresh spends quota). DB lane.
+"""
+
+from datetime import datetime, timedelta, timezone
+
+import pytest
+
+from app.models import Channel, Subscription
+from app.routes import channels
+from app.sync import subscriptions
+from app.sync.subscriptions import DETAILS_MAX_AGE, import_subscriptions
+
+NOW = datetime.now(timezone.utc)
+FRESH = NOW - timedelta(days=1)
+STALE = NOW - DETAILS_MAX_AGE - timedelta(hours=1)
+
+
+def _item(cid: str) -> dict:
+    return {
+        "id": cid,
+        "snippet": {"title": f"{cid} renamed", "publishedAt": "2010-01-01T00:00:00Z"},
+        "brandingSettings": {"image": {"bannerExternalUrl": f"https://yt3.example/{cid}-new"}},
+    }
+
+
+def _youtube(asked: list[list[str]], subscribed: list[str]):
+    class _Client:
+        def __init__(self, db, user):
+            pass
+
+        def __enter__(self):
+            return self
+
+        def __exit__(self, *exc):
+            return False
+
+        def iter_subscriptions(self):
+            for cid in subscribed:
+                yield {"channel_id": cid, "title": cid, "yt_subscription_id": f"sub-{cid}"}
+
+        def get_channels(self, ids):
+            asked.append(sorted(ids))
+            return [_item(cid) for cid in ids]
+
+    return _Client
+
+
+def _channel(db, cid: str, synced: datetime | None) -> None:
+    db.add(
+        Channel(
+            id=cid,
+            title=cid,
+            banner_url=f"https://yt3.example/{cid}-old",
+            published_at=datetime(2010, 1, 1, tzinfo=timezone.utc),
+            details_synced_at=synced,
+        )
+    )
+
+
+def test_the_import_refreshes_stale_and_missing_details_and_leaves_fresh_ones(
+    db, make_user, monkeypatch
+):
+    user = make_user()
+    _channel(db, "UCfresh", FRESH)
+    _channel(db, "UCstale", STALE)
+    _channel(db, "UCnever", None)
+    db.flush()
+    for cid in ("UCfresh", "UCstale", "UCnever"):
+        db.add(Subscription(user_id=user.id, channel_id=cid))
+    db.commit()
+    asked: list[list[str]] = []
+    monkeypatch.setattr(
+        subscriptions, "YouTubeClient", _youtube(asked, ["UCfresh", "UCstale", "UCnever"])
+    )
+
+    result = import_subscriptions(db, user)
+
+    assert asked == [["UCnever", "UCstale"]]
+    assert result["channels_detailed"] == 2
+    stale = db.get(Channel, "UCstale")
+    assert stale is not None
+    assert stale.banner_url == "https://yt3.example/UCstale-new"
+    assert stale.details_synced_at is not None and stale.details_synced_at > STALE
+    fresh = db.get(Channel, "UCfresh")
+    assert fresh is not None and fresh.banner_url == "https://yt3.example/UCfresh-old"
+
+
+@pytest.mark.parametrize(("synced", "refreshed"), [(STALE, True), (FRESH, False)])
+def test_the_channel_page_refreshes_a_stale_channel_only(
+    db, make_user, monkeypatch, synced, refreshed
+):
+    user = make_user()
+    _channel(db, "UCpage", synced)
+    db.commit()
+    asked: list[list[str]] = []
+    monkeypatch.setattr(channels, "YouTubeClient", _youtube(asked, []))
+
+    body = channels.channel_detail("UCpage", user=user, db=db)
+
+    assert asked == ([["UCpage"]] if refreshed else [])
+    suffix = "new" if refreshed else "old"
+    assert body["banner_url"] == f"https://yt3.example/UCpage-{suffix}"