Document provider integration contracts

docs/integrations/openmeteo.md

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+# Open-Meteo Integration Notes
+
+## Purpose
+
+This document describes the Open-Meteo API usage currently implemented by
+`weatherfeeder`. It is for developers and operators maintaining Open-Meteo
+source URLs, normalizers, fixtures, and tests.
+
+General config syntax belongs in [configuration](../config.md). Emitted JSON
+events are documented in [event wire contract](events.md).
+
+## Implemented Drivers
+
+| Driver | Kind | Raw schema | Canonical schema |
+| --- | --- | --- | --- |
+| `openmeteo_observation` | `observation` | `raw.openmeteo.current.v1` | `weather.observation.v1` |
+| `openmeteo_forecast` | `forecast` | `raw.openmeteo.hourly.forecast.v1` | `weather.forecast.v1` |
+
+## Config Requirements
+
+Both drivers require HTTP source params:
+
+- `url`
+- `user_agent`
+
+The shared HTTP helper also accepts `conditional`, `http_timeout`, and
+`http_response_body_limit_bytes`. Conditional requests are enabled by default;
+an upstream `304 Not Modified` response emits no event for that poll.
+
+## Upstream Shapes Used
+
+`openmeteo_observation` expects a JSON response with top-level location/timezone
+metadata and a `current` object. The normalizer uses:
+
+- `latitude`, `longitude`, `timezone`, `utc_offset_seconds`;
+- `current.time`;
+- current temperature, apparent temperature, relative humidity, weather code,
+  wind speed/direction/gusts, pressure, and `is_day`.
+
+`openmeteo_forecast` expects top-level location/timezone metadata and an
+array-oriented `hourly` object. The normalizer uses:
+
+- `hourly.time`;
+- hourly temperature, apparent temperature, dew point, relative humidity,
+  precipitation probability, precipitation amount, snowfall, weather code,
+  pressure, wind speed/direction/gusts, `is_day`, cloud cover, visibility, and
+  UV index.
+
+Open-Meteo field presence is allowed to vary. Missing optional arrays produce
+nil canonical fields for the affected periods.
+
+## Accept Header
+
+Open-Meteo sources request:
+
+```text
+application/json
+```
+
+## Time Handling
+
+Open-Meteo timestamps often omit an explicit offset. The provider helper parses
+times by using the returned `timezone` or `utc_offset_seconds` when needed.
+
+Observation source events set `effective_at` from `current.time` when it can be
+parsed. Hourly forecast source events prefer `current.time`, then the first
+non-empty `hourly.time` entry.
+
+The hourly forecast normalizer sets canonical `issuedAt` from the incoming event
+`emitted_at` when present, otherwise from the first hourly period start.
+Normalized forecast `effective_at` matches `issuedAt`.
+
+## Mapping Notes
+
+Open-Meteo is not a station feed. Weatherfeeder synthesizes canonical
+station/location IDs from latitude and longitude when both are available.
+
+Open-Meteo weather codes are WMO codes and are treated as authoritative.
+Canonical text is derived from the WMO code and day/night hint.
+
+Wind speed and gust fields are treated as kilometers per hour. Pressure values
+are treated as hPa and converted to Pa. Snowfall values are treated as
+centimeters and converted to millimeters.
+
+Hourly forecast period end time is the next period start. The last period uses
+the previous interval length, or one hour when there is no previous interval.
+
+## Failure Behavior
+
+Constructor validation failures stop daemon startup. Polling failures are
+returned to the scheduler. Metadata decoding failures in sources still allow raw
+payload emission when the HTTP response itself succeeded.
+
+Normalization fails when required time data is missing or invalid, such as an
+empty `hourly.time` array for hourly forecasts.
+
+## Tests To Inspect
+
+- `internal/sources/openmeteo/source_test.go`
+- `internal/normalizers/openmeteo/*_test.go`
+- `internal/providers/openmeteo/*_test.go`