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/app/worker.py

@@ -29,6 +29,7 @@ from datetime import datetime, timezone
 from sqlalchemy import text
 
 from app import logs
+from app import logsink
 from app import sysconfig
 from app.config import settings
 from app.db import SessionLocal
@@ -519,6 +520,10 @@ def main() -> None:
         if _stop.is_set():
             return  # asked to shut down while waiting — that IS a clean exit
         raise SystemExit(1)
+    # After the wait, not before: the DB is reachable now. The wait checks `download_jobs`, not
+    # `log_events`, so on the deploy that first adds the table the worker may start before the API
+    # has migrated — those few seconds of batches fail and say so on stderr; nothing else breaks.
+    logsink.start()
     _recover_orphans()
     n = max(1, settings.download_worker_concurrency)
     log.info("Download worker started (concurrency=%d, root=%s).", n, settings.download_root)