v0.18.0 — Similar opportunities + personal notes on saved items

Tier-2 market-research features, following on from v0.17.0's countdowns/
calendar/status-tracking:

• Similar opportunities on the detail page — same-category, live,
  soonest-deadline-first recommendations.
• Personal notes on saved items — free-text field for context the
  application status alone can't capture (e.g. "need 2 recommenders").

Migration: 20260705010000_saved_notes.sql.