feat(seed): conditionally seed demo locations when their R2 objects exist

[AlaskanTuna]

Closes the prisma/seed.ts half of #10. The frontend half landed in #36.

Why it is conditional rather than a plain upsert

I argued against seeding at all in #36, and that reasoning still holds: a seed can write Location rows but cannot create their R2 objects. A row marked ready whose rgbImageUrl/maskPath/monthlyFluxPath point at objects a bucket does not contain fails later and more confusingly than a clean cache miss — the app thinks it has a cached location and then cannot render it.

So seedDemoLocations() writes a row only when it can prove the assets exist. Per location, in order:

1. Row already within ±0.0001° → skip (idempotent).
2. No rescued source row with non-null buildingInsightsJson in this database → skip. "the seed never fabricates Solar API payloads." In a fresh environment all four take this path.
3. Rescued row missing storage paths → skip, naming the missing fields.
4. HeadObject probe 404s on any path → skip, listing the missing objects, so the cache misses cleanly.
5. All probes pass → create at the exact demo coordinates, cloning the payload from the rescued row.

Missing R2 env skips the whole phase with one log line, and the demo phase is wrapped so it can never take the required TariffConfig seed down with it.

Idempotency is find-then-create keyed on the same 0.0001 tolerance resolveLocation uses — Location has no unique constraint on lat/lng, so upsert would not work.

Known wart

The demo coordinates are duplicated from frontend/src/lib/demoLocations.ts. A clean import was not possible: the seed runs as plain Node with type stripping and no path aliases, and reaching into frontend/src would drag React types across the workspace boundary. The constant carries a comment naming the other file and the drift risk.

Worth hoisting into shared/ later — noting that shared builds to JS, so the seed would need a runtime path into that build. Drift here is not cosmetic: these coordinates only work because they match cached rows within 11 m, so a stale copy silently converts a cache hit into a Solar API call.

Verified

tsc --noEmit -p backend/tsconfig.json exit 0 · node --check on both files exit 0 · import smoke test exports seedDemoLocations · Prettier clean on both files.

Not executed against any database — the HeadObject path is untested against real credentials. Also note prisma/ is in no tsconfig, so the backend typecheck does not actually cover these files; a direct strict check was run instead.

Implemented by the qwen3.8-max peer session; verified here.

https://claude.ai/code/session_01URDpaxhXyPLWnUQtvowNyM

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