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/tests/test_logs.py

@@ -219,6 +219,14 @@ def _client() -> TestClient:
             raise HTTPException(status_code=400, detail={"code": "bad_cursor"})
         return {"ok": True}
 
+    @app.get("/api/messages/unread_count")
+    def _unread():
+        return {"count": 0}
+
+    @app.post("/api/messages/unread_count")
+    def _unread_write():
+        return {"ok": True}
+
     @app.get("/boom")
     def _boom():
         raise RuntimeError("kaboom")
@@ -323,6 +331,30 @@ def test_the_log_consoles_own_reads_are_debug_but_its_failures_are_not(sink):
     assert sink.objects()[-1]["level"] == "WARNING"
 
 
+def test_a_polled_read_is_debug_a_write_to_the_same_route_is_not(sink):
+    # One open tab polled the unread counts, /api/me and the download index every 30 s: at INFO
+    # that was ~95% of the stored log on prod.
+    http = logging.getLogger("siftlode.http")
+    http.setLevel(logs.TRACE)
+    client = _client()
+    client.get("/api/messages/unread_count")
+    assert sink.objects()[-1]["level"] == "DEBUG"
+    client.post("/api/messages/unread_count")
+    assert sink.objects()[-1]["level"] == "INFO"
+    client.get("/api/videos/abc")
+    assert sink.objects()[-1]["level"] == "INFO"
+
+
+def test_every_polled_route_is_a_route_the_app_has():
+    # The list names route TEMPLATES; one renamed on the router side would silently fall back to
+    # INFO, so the list is held to the app's own routes.
+    from app.main import app
+
+    # The schema, not `app.routes`: FastAPI 0.141 keeps an included router behind a wrapper there.
+    gets = {path for path, ops in app.openapi()["paths"].items() if "get" in ops}
+    assert logs._POLLED_ROUTES - gets == set()
+
+
 def test_a_failing_request_is_logged_by_status_and_an_unhandled_error_is_logged_at_all(sink):
     client = _client()
     client.get("/api/videos/missing/deeper")  # 404 from the router