Stale job detection — URL validation, auto-expiry, user feedback

[TheTrueAI]

Phase: 3 — Growth & Scale (Quality)

Reduce stale/dead job listings:

• HEAD request validation — check if apply URLs return 200 before including a job
• Track job first_seen date in DB — auto-expire jobs older than 45 days
• Add "This job no longer exists" user feedback button — mark as expired in DB
• Add portal quality scoring — down-rank sources with high dead-link rates

This is a known issue: some jobs found by SerpAPI through aggregators no longer exist.