feat: environment-aware datasette base URL

[stefanb]

Supersedes #604 (closed; it was stacked on #603 — this PR targets main directly and carries no CLAUDE.md changes).

Problem

Every website environment queried the stage datasette: https://stage-data.podnebnik.org was baked into the JS bundle (hardcoded in two of the three islands). Production served stage data, local instances couldn't use a local datasette, and previews were implicitly tied to stage.

What this does

Shared resolver — code/datasette-base.ts, consumed by all three islands (ERA5 api.ts, legacy ali-je-vroce/constants.ts, temperatura/heatmaps.jsx). Precedence:

1. VITE_DATASETTE_URL build-time override (compose → 127.0.0.1:8001; manual dev → :8010; preview builds → per-PR)
2. Fixture mode → the recorded stage base (2,087 URL-keyed fixtures stay valid)
3. podnebnik.org / stage.podnebnik.org → same-origin /data — the gateway already routes each host's /data prefix to its own datasette, so the single promoted image reaches prod data on prod and stage data on stage, with no CORS
4. Fallback → stage base (Node/tests, localhost, previews without their own datasette)

Local — docker compose wires the website dev server to the local datasette container.

Previews (preview-deploy label) — docker-preview.yaml detects whether the PR touches the datasette path set (same list as docker-data.yaml; PR-files API, fail-closed):

• touches data → builds ghcr.io/podnebnik/data:pr-<N> (db-export generation + multi-arch runtime in one job), bakes /data into the website bundle, and applies the preview-data label after the image push
• doesn't → bakes the stage base and retracts a stale preview-data label

The Helm chart gains a previewDatasette component (Deployment/Service + /data HTTPRoute rewrite mirroring the live stage/prod route shape; templated rollout revision so each push re-pulls the mutable pr-<N> tag). Chart 1.1.0 → 1.2.0; default render unchanged.

⚠️ Infrastructure prerequisite (sledilnik/infrastructure)

The per-PR-preview half needs the podnebnik-website-preview ApplicationSet to enable the component when the PR carries the preview-data label, e.g. in the generator/template:

previewDatasette:
  enabled: true          # gate on the preview-data PR label
  image:
    tag: "pr-{{ .number }}"
  revision: "{{ .head_sha }}"   # forces re-pull of the mutable tag on each push

Everything else (prod/stage/local) works without any infra change. Ops note: prod pages will start reading data.podnebnik.org, whose image tag is pinned by hand in the infra repo — promote the data image alongside the website when relevant.

Verification

• yarn typecheck:gate — OK (zero-error allowlist holds)
• yarn test — 100/100, fixture base guard passes (resolver + new VITE_DATASETTE_URL="" pin land atomically — "" under the old ?? logic would have broken the suite)
• yarn snapshot:check — byte-identical, 0 fixture misses (the resolver keeps import.meta.env.* as literal member expressions so the snapshot define block still applies)
• helm template — preview render shows datasette Deployment (pr-999, revision templated) + /data route; default render identical to before
• Local e2e: compose website dev server injects VITE_DATASETTE_URL=http://127.0.0.1:8001; local datasette verified serving climate-si

🤖 Generated with Claude Code

[ntadej]

Before we merge we should figure out how to auto-deploy data to prod, because we do daily updates. Maybe an automated MR should be triggered like for daily refresh?

Will look at the MR in details later.