Show the snapshot archiving process on the status page - #21

The status page listed every background job except the archiver, so the
only way to tell whether snapshots were still draining to R2 was to open
bullboard or query the database by hand. That gap mattered during the
failures documented in 0b44cf7, where archive jobs were dying on
connection loss and nothing surfaced it.

Adds an "Archiving snapshots" row to the job list, plus an Archiving
section with the retention window, the age of the oldest snapshot still
in Postgres, whether that snapshot is past retention, and the queue's
failed job count.

Oldest snapshot is read with ORDER BY id ASC LIMIT 1, a primary key
lookup. Counting rows past the retention cutoff would be a sequential
scan, since GameServerSnapshot has no index on createdAt, and it would be
slowest exactly when the table is backlogged and the page is being
looked at. Oldest-snapshot age carries the same signal.

Two supporting changes:

- SNAPSHOT_RETENTION_HOURS moves from the worker to libs/teerank so the
  frontend renders the same number the worker enforces. Neither container
  sets it, so both still default to 48, but an override now has to be set
  on both.
- removeOnComplete.age on the archive queue goes 10 minutes -> 6 hours.
  The job is scheduled every 10 minutes, so the completion record expired
  at roughly the schedule cadence and the last run would intermittently
  disappear, reading as Down while the archiver was healthy.

The archiving row uses a 30 minute staleness threshold rather than the 10
minutes the other jobs use, because the job is scheduled every 10 minutes
and is allowed to run for 5.

Co-authored-by: Claude Opus 5 (1M context) <[email protected]>