Add basemap tile storage: Bicep infra and GPKG extraction script

[Copilot]

Serve Lantmäteriet topowebb raster tiles directly to MapLibre from Azure Blob Storage — no Azure Function proxy needed. Tiles are extracted once from a GeoPackage and stored at {z}/{x}/{y} paths with public anonymous read access.

infra/basemap.bicep

• Standard LRS StorageV2 account with allowBlobPublicAccess: true
• CORS: GET/HEAD/OPTIONS from any origin, 24h max-age
• basemap-tiles container with publicAccess: Blob
• Outputs tilesBaseUrl for direct use in MapLibre config

Backend/Scripts/extract-gpkg-tiles.py

One-time population script (pure sqlite3, no GDAL):

• Reads (zoom_level, tile_column, tile_row, tile_data) from the GPKG tile table
• TMS → OSM Y-flip: osm_y = (2^z - 1) - gpkg_tile_row
• Detects Content-Type from magic bytes (PNG/JPEG/WebP); sets Cache-Control: public, max-age=86400
• 16 parallel upload threads; skips existing blobs unless --overwrite

python extract-gpkg-tiles.py \
    --gpkg topowebb.gpkg \
    --connection-string "DefaultEndpointsProtocol=https;AccountName=...;..." \
    --zoom 0,1,2,3,4,5,6,7,8,9,10,11,12

MapLibre tile source:

{
  "type": "raster",
  "tiles": ["https://<account>.blob.core.windows.net/basemap-tiles/{z}/{x}/{y}"],
  "tileSize": 256
}