M0-3b — DB-backed SpotService reads (fail-fast)

[kylejb]

Epic: #198 (E1)

Wire the running app to read from Postgres instead of the in-memory seed + live NDBC. Depends on #210 (ingestion populating observations).

Scope

• cmd/api: build a pgx pool + store.New from env.GetString("DATABASE_URL"); fail fast if unset/unreachable (the app is DB-backed).
• SpotService (internal/http/spot.go): read spots via store.ListSpots/GetSpot and latest conditions via store.LatestObservation — instead of spot.All()/spot.ByID() + live noaa.Realtime.
• Map store.Observation → proto Conditions (mirroring the current toProtoConditions).

Acceptance

• cmd/api requires DATABASE_URL; refuses to start without a reachable DB
• GetSpot/ListSpots return DB-backed data; latest observation comes from observations
• handler tests updated (stub store)

Split from #210.