Adding a DWD product

Each DWD product is one coordinator subclass in products.py. Adding one touches a handful of places, in this order.

Checklist

1. Subclass a base in products.py:
   • WholeFileProduct for one file per release, decoded whole (HymecNG, RADOLAN);
   • RadvorProduct for a RADVOR tar archive (RS, RV);
   • BaseProductUpdateCoordinator itself only if neither fits.
2. Set the class constants: PRODUCT_KEY, PRODUCT_LABEL, RELEASE_INTERVAL, RELEASE_DELAY, RELEASE_OFFSET and OVERDUE_GRACE (spelled out even when it is DEFAULT_OVERDUE_GRACE; a test enforces it), COVERAGE_KEYS (the sub-keys that are None without data for the cell; a test checks every product has them), and MAX_FAST_POLL_INTERVAL if the default does not fit. See below.
3. Add the URL pattern to radar/urls.py; _get_url derives the URL from PRODUCT_KEY.
   • A WholeFileProduct implements the blocking _parse(content, ts); the base downloads the file and runs it in the executor.
   • A RadvorProduct implements _fetch_and_parse.
   • Keep _fetch_and_parse free of side effects; anything else goes in _async_on_new_release.
4. The grid comes from each file's /where group (radar/grid.py), so a product on another polar-stereographic grid needs nothing more; one in another projection needs that projection in grid.py.
5. Add sensor descriptions in sensor.py (a new *_SENSORS tuple).
6. Register the class in the PRODUCT_CLASSES tuple of __init__.py.
7. Register the sensors in async_setup_entry of sensor.py, and add each translation_key's name to translations/en.json and translations/de.json. The two files must keep identical keys; a test checks it.
8. Add it to PRODUCTS in scripts/check_release_delay.py, so its RELEASE_DELAY is checked against what DWD actually does.
9. Document it: an entity page in website/docs/entities/ and its German translation, and the tables in the introduction, the entity overview and Release timing.

Picking the release constants

• RELEASE_INTERVAL and RELEASE_OFFSET describe the nominal times: every 5 minutes at :00, :05, … is an interval of 5 minutes and no offset; hourly at :50 is an interval of 1 hour and an offset of 50 minutes.
• RELEASE_DELAY is how long after its nominal time a file is on Open Data. Measure it from the Last-Modified headers of recent files (that is what scripts/check_release_delay.py does) and take a value at or above the mean: fetching before DWD publishes costs a failed request and a retry every time.
• OVERDUE_GRACE is how long DWD is given to publish an overdue release before the sensors go stale. It must not be a whole multiple of RELEASE_INTERVAL, or the deadline coincides with a fetch; a test enforces that per product.
• USE_LOCAL_TIME is for a product whose nominal times are on local time, like the daily sf_2350.

Naming the entities

Follow the naming rules in CLAUDE.md and ADR 0001: a window in the name only when it is part of the value, last / next / calendar periods, units that follow the product. Never give a new entity an id a released version gave another one; tests/entity_ids.py records them.