weather-voodoo: adopt OpenWeather One Call 4.0 — alerts now, 15-min nowcast later

[radumarias]

Context

weather-voodoo currently calls the One Call 3.0 endpoint (hardcoded in weather-voodoo/src/lib/server/openweather.ts). OpenWeather now recommends the Timeline-Based Weather API — One Call 4.0 for all new integrations (proprietary OWHL model, 10-min refresh, minutely + 15-min forecasts, 48h hourly, alerts).

Audit of what we actually consume today:

Data	Source	Status
Forecast, wind, rain, waves, UV	Open-Meteo (free)	working
AQI	OpenWeather /data/2.5/air_pollution (free tier)	working
Government weather alerts	OpenWeather One Call	blocked — needs One Call subscription (401 today)
OWM UV (uvIndexCurrent / uvIndexHourly)	extracted server-side	dead code — no client consumer; UV comes from Open-Meteo

One Call 4.0 cannot replace Open-Meteo as the forecast backbone: it has no marine/wave data (sea mode depends on it) and its hourly horizon is 48h while the today/tomorrow/day-after UI needs ~72h. Open-Meteo (ECMWF/ICON/regional models) stays primary; OpenWeather's unique value for us is alerts — and later, nowcasting.

Proposed changes

1. Migrate openweather.ts from 3.0 → 4.0
   • Swap ONECALL_URL to the 4.0 endpoint; verify alerts[] (and hourly field names) against the 4.0 response schema.
   • Update the misleading error copy ("3.0 requires paid plan").
   • Subscription note: "One Call by Call" — 1,000 free calls/day; default cap is 2,000, set it to 1,000 in account settings to guarantee $0.
2. Light up the alerts panel — already wired in FixedView.svelte; it just needs the subscription + working endpoint.
3. Cleanup while there
   • Remove the unused uvIndexCurrent/uvIndexHourly extraction (UV is Open-Meteo's).
   • Optional: move AQI to Open-Meteo's free air-quality API (CAMS) so OpenWeather serves alerts only — fewer paid calls, one less legacy 2.5 endpoint.

Future (separate follow-up): "leaving right now" widget

4.0's minutely (60 min) + 15-min (48 h) nowcast is only valuable for near-term decisions ("do I leave now or in 20 minutes to dodge this rain cell"). The app's whole pipeline — ForecastHour, fusion, 3h blocks, trip-window scoring, share-link slot keys — is hourly by design, and converting it to 15-min steps is a large change for marginal benefit.

Instead: a small standalone "leaving right now" widget fed directly from the 15-min/minutely series (next 1–2 h precipitation/wind at fine grain), rendered independently of the hourly table. Complements the existing "Right now" trip-duration option. Any new UI text must follow the i18n workflow in weather-voodoo/CLAUDE.md.

Acceptance

• /api/owm?kind=onecall returns available: true with alerts via One Call 4.0 (given a subscribed key)
• Alerts render on the fixed-location tab; graceful {available:false} degradation unchanged without a key
• Dead UV extraction removed
• (Optional) AQI served keyless from Open-Meteo air-quality API
• Nowcast widget scoped as its own issue when/if wanted

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