feat: store warnings and request lines in log_events

The log now has a stored, queryable copy for the diagnostics console:
every WARNING and above plus the request line of every request, kept
for log_retention_days (default 7, no "forever") and pruned hourly by
the new log_gc job.

Rows are built on the logging thread from the same redacted fields as
the stdout line, queued without blocking and written in batches by one
background thread on its own connection. A full queue or a failed write
drops and reports instead of retrying. Flood control keeps 30 of any
one message per minute plus a count of the rest; anonymous request
lines are capped at 60 a minute per client, and the client address is
never stored. Deleting an account deletes its stored events.

Measured on prod first: a YouTube RSS outage wrote 11k warnings in four
hours, one per channel per poll. The poll now writes one warning with
per-reason counts and a few examples, and a failure on our side gets
one traceback per poll instead of one per channel.

backend/tests/conftest.py

@@ -57,6 +57,10 @@ def _db_reachable() -> bool:
 
 _DB_REACHABLE = _db_reachable()
 
+# No background log writer in tests: every TestClient would otherwise start one on app startup and
+# write into tables the fixtures truncate between tests. The sink's own tests drive it explicitly.
+settings.log_db_sink = False
+
 
 @pytest.fixture(scope="session")
 def _schema():