[weather-voodoo] Watch a place & notify on a good window (Web Push)

[radumarias]

Right now the user has to remember to come back and check. A "ping me when the window opens" feature is the only thing that turns this from a tool you use once into one you keep installed.

Goal

Let the user subscribe a saved location + threshold and receive a Web Push notification when a matching window appears within the forecast horizon.

Example subscription

Phi Phi · sea · 4h window · score ≥ 80 · within next 3 days

Scope

• UI: "🔔 Watch this place" button on a forecast view; opens a small modal to set duration, mode, score threshold, lookahead.
• Web Push permission request and PushSubscription capture.
• Server side:
  • Storage for subscriptions (Vercel KV is gone; use Marketplace Postgres / Upstash Redis).
  • Cron job (crons in vercel.ts) every 30 min that re-fetches each subscribed location, runs the same findBestWindows logic, and emits a push if a window passes the threshold and hasn't been notified for this 24h slot already.
• VAPID keys + web-push library on the server.
• Unsubscribe link in each notification.

Out of scope

• iOS < 16.4 doesn't support Web Push; treat as graceful degradation.

Effort

Large. Touches infra (cron + KV/Postgres), client (notification permission UX), and a non-trivial scheduler. Worth doing only after #15 (saved locations) and #19 (tides if marine-focused).