v0.19.0 — Eligibility profile matching + instant new-match alerts

Real Tier-2 market-research features, the honest zero-cost version of the
roadmap's "for you" ranking (§3d):

• Eligibility profile: one-time study-level + region preference, badged
  as "✓ Matches you" on the feed and detail page via simple tag overlap
  against audience_tags/region_tags — no ML, no new API cost.
• Instant new-match alerts: a daily cron checks each followed filter for
  opportunities approved since its last check, and emails/pushes one
  combined digest per user — no per-item spam even if someone follows
  several filters.

Also extracted the tag-overlap matching logic (previously duplicated across
the digest and calendar.ics routes) into a shared helper, now used by both
plus the new alerts module.

Migrations: 20260705030000_eligibility_profile.sql,
20260705040000_filter_alert_watermark.sql.