- Adopt an opinionated Event.ID policy across sources:
- use upstream-provided ID when available
- otherwise derive a stable ID from Source:EffectiveAt (RFC3339Nano, UTC)
- fall back to Source:EmittedAt when EffectiveAt is unavailable
- Add common/id helper to centralize ID selection logic and keep sources consistent
- Simplify common event construction by collapsing SingleRawEventAt/SingleRawEvent
into a single explicit SingleRawEvent helper (emittedAt passed in)
- Update NWS/Open-Meteo/OpenWeather observation sources to:
- compute EffectiveAt first
- generate IDs via the shared helper
- build envelopes via the unified SingleRawEvent helper
- Improve determinism and dedupe-friendliness without changing schemas or payloads
- Require params.user_agent for all HTTP sources (uniform config across providers)
- Add common.RequireHTTPSourceConfig() to validate name/url/user_agent in one call
- Add common.NewHTTPClient() with DefaultHTTPTimeout for consistent client setup
- Add common.SingleRawEvent() to centralize event envelope construction + validation
- Refactor NWS/Open-Meteo/OpenWeather observation sources to use new helpers
