Document the Brīdinājumi workflow and projection calibration

docs/ARCHITECTURE.md

@@ -30,6 +30,7 @@ Staging currently exercises the PostgreSQL → API → frontend path with synthe
 - Serves JSON API routes and the built frontend from `web/dist`.
 - Queries and aggregates station observations in PostgreSQL.
 - Contains ingestion/parsing code for LVGMC station and forecast data.
+- Fetches LVĢMC open hydrometeorological-warning metadata and polygon coordinates on request, with a 5-minute in-memory cache and stale-data fallback if the upstream call fails.
 - Contains HARMONIE discovery, download, GRIB parsing, and rendering support.
 - Starts scheduled ingestion/cleanup tasks unless `ENABLE_SCHEDULED_JOBS=false`.
 
@@ -82,6 +83,12 @@ The renderer attaches each normalized 256×256 weather image to the measured edg
 
 Monda Regular and Bold are bundled locally and used by the interface and generated text. Weather symbols are bundled as independent transparent PNG assets.
 
+### Brīdinājumi
+
+Brīdinājumi renders the current LVĢMC warning polygons over the same `map_1920x1080`/`map_3840x1440` backgrounds used by Kartes, plus a production border-outline overlay drawn last so it stays crisp above the softened data layer. Warning severities are blurred and filled onto an offscreen layer before compositing, giving the broadcast graphic a feathered rather than hard-edged warning area.
+
+The backend returns warning polygons as raw latitude/longitude points; the frontend projects them to pixel space with a per-resolution affine transform (`x = a·lon + b·lat + c`, `y = d·lon + e·lat + f`). Each transform is a least-squares fit of well-spread Latvian cities' real-world coordinates against the pixel positions already validated for these same map backgrounds by Kartes/Faktiskā (`cityCoords.ts` + `mapConsts.ts`), rather than an independently eyeballed calibration. An earlier 4-corner bounding-box version of this projection could not represent the source artwork's rotation/shear and drifted up to ~200px on the 3840 canvas; the affine fit lands within ~35px.
+
 ## What the repository does not provide by itself
 
 - Authorized provider accounts and credentials.