feat: give the sync a memory, so it stops paying for the same listing twice

Nothing in this tool had any memory: every invocation started from zero
and would happily re-enumerate a profile it had listed minutes earlier.
That is what suspended the account -- the listing passes, not the
downloads -- and an aborted run re-enumerating five profiles on restart
was a large part of the bill.

Three changes, in order of how much they save:

- Seeding is now a one-time bootstrap per source. After the first
  successful sync the archive DB records everything gallery-dl has seen,
  so the source is never probed again. A second full sync costs roughly
  half what the first did.
- Stories never seed at all. A story cannot be in the archive before it
  is fetched, so there is nothing to seed from, and probing would double
  the cost of the cheapest surface we have.
- A source fetched within --min-interval (20h) is refused, and listing
  results are cached for --probe-ttl (24h), so a restart mid-run is free
  rather than a repeat. --force overrides both.

--only replaces --no-stories and takes any subset of the surfaces, which
is what makes a daily stories-only run possible: one source per profile,
no seeding, a handful of requests. Everything else stays monthly.

Tested with stdlib unittest -- no new dependencies, and it runs anywhere
the sync does. The cases include the aborted-restart scenario, which now
plans zero work.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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