fix(logs): keep polling reads and a channel-less account out of the warnings

The reads the frontend polls (unread counts, /api/me, the download index,
sync status, subtitle progress) are logged at DEBUG: at INFO they were ~95%
of the stored log on prod. A scheduler job failure and a playlist sync
failure carry event codes, and a Google account without a YouTube channel
has an empty playlist list instead of a warning every six hours.

backend/app/youtube/client.py

@@ -6,6 +6,7 @@ so they don't depend on a specific user's token; subscriptions.list?mine=true al
 uses OAuth.
 """
 
+import logging
 from collections.abc import Iterator
 from datetime import datetime, timedelta, timezone
 
@@ -50,7 +51,9 @@ def _error_reason(resp: httpx.Response) -> str:
     return reasons or "<none>"
 
 
-def _upstream_failure(method: str, path: str, resp: httpx.Response) -> YouTubeError:
+def _upstream_failure(
+    method: str, path: str, resp: httpx.Response, expected: frozenset[str] = frozenset()
+) -> YouTubeError:
     """Report one upstream failure and build the error to raise. Every YouTube response that is
     not a success goes through here — the Data API's reads, writes and deletes and the OAuth
     token refresh — so what may appear in a log line cannot drift apart between them, and the
@@ -59,7 +62,11 @@ def _upstream_failure(method: str, path: str, resp: httpx.Response) -> YouTubeEr
     response at all is `_send`'s, with the reason `transport`.) The body is logged only under
     the requesting user's own Teljes consent: it is the request quoted back."""
     reason = _error_reason(resp)
-    log.warning(
+    # A reason the caller named as an answer rather than a failure (`channelNotFound` on a `mine`
+    # read: the account simply has no channel) is still raised, but it is not a warning — it came
+    # back every six hours for an account that will never have one.
+    log.log(
+        logging.DEBUG if reason in expected else logging.WARNING,
         "YouTube %s %s -> %s (%s)",
         method,
         path,
@@ -83,6 +90,10 @@ def _upstream_failure(method: str, path: str, resp: httpx.Response) -> YouTubeEr
     return YouTubeError(f"{method} {path} -> {resp.status_code}: {reason}", reason=reason)
 
 
+# The `mine` reads' answer for a Google account that has no YouTube channel.
+_NO_CHANNEL = frozenset({"channelNotFound"})
+
+
 def _chunks(items: list, size: int) -> Iterator[list]:
     for i in range(0, len(items), size):
         yield items[i : i + size]
@@ -167,7 +178,14 @@ class YouTubeClient:
         return access
 
     # --- core request ---
-    def _get(self, path: str, params: dict, cost: int = 1, allow_key: bool = True) -> dict:
+    def _get(
+        self,
+        path: str,
+        params: dict,
+        cost: int = 1,
+        allow_key: bool = True,
+        expected: frozenset[str] = frozenset(),
+    ) -> dict:
         p = dict(params)
         headers = {}
         api_key = sysconfig.get_str(self.db, "youtube_api_key")
@@ -178,7 +196,7 @@ class YouTubeClient:
         resp = self._send("GET", f"{API_BASE}/{path}", params=p, headers=headers)
         quota.record_usage(cost)
         if resp.status_code != 200:
-            raise _upstream_failure("GET", path, resp)
+            raise _upstream_failure("GET", path, resp, expected)
         return resp.json()
 
     # --- endpoints ---
@@ -218,7 +236,14 @@ class YouTubeClient:
             params = {"part": "snippet,contentDetails", "mine": "true", "maxResults": 50}
             if page_token:
                 params["pageToken"] = page_token
-            data = self._get("playlists", params, cost=1, allow_key=False)
+            try:
+                data = self._get("playlists", params, cost=1, allow_key=False, expected=_NO_CHANNEL)
+            except YouTubeError as exc:
+                # A Google account without a YouTube channel has no playlists, and YouTube says
+                # so as a 404. That is an empty list, not a failed sync.
+                if exc.reason in _NO_CHANNEL:
+                    return
+                raise
             for item in data.get("items", []):
                 sn = item.get("snippet", {})
                 yield {