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/downloads/runner.py

@@ -18,6 +18,7 @@ import subprocess
 import sys
 
 from app import logs
+from app import logsink
 from app.config import settings
 from app.db import SessionLocal
 from app.downloads import edit as editmod
@@ -62,6 +63,7 @@ def main() -> None:
     # This whole process exists to run ONE job — tag every line it prints with it, so the child's
     # output joins up with the worker's and the API's view of the same download.
     logs.set_job(job_id)
+    logsink.start()  # flushed at exit (atexit) — this process has no shutdown hook of its own
 
     with SessionLocal() as db:
         job = db.get(DownloadJob, job_id)