Support cropped (bbox/extent) exports and raster (.tif) caching #14

Problem

koop cannot currently download a clipped layer or cache a raster, which blocks using it to acquire large rasters (e.g. national LINZ DEMs) for a small AOI.

Two gaps:

1. No AOI/extent cropping. post_export (post_export.py) builds the request body with only formats + items (+ optional named tiles). ExportRequestSchema.extent (schemas.py) is typed extent: None = None and never set, so every export is the whole layer. The PolygonExtent/MultiPolygonExtent schemas and the is_cropped response field are scaffolded but not wired through. A whole national DEM is far too large to download for a small area of interest.

2. GeoPackage-only caching path. get_latest_layer (get_latest.py) hardcodes …_{hash}.gpkg and glob("*.gpkg"), raising "Should only be one geopackage per export" if the export is not exactly one geopackage. A raster (grid) layer exports a .tif, so the raster path errors — despite the README advertising raster (GeoTIFF) support.

Proposed change

1. Thread an optional extent (a GeoJSON Polygon/MultiPolygon, EPSG:4326 per the Koordinates API) through get_layer_from_id → download_layer → create_export → post_export → ExportRequestSchema.extent, so callers can request a cropped export.
2. Make get_latest_layer (and download_layer) raster-aware: when the layer kind is grid, expect/cache a .tif (…_{hash}.tif, glob *.tif) instead of .gpkg. Drive the extension off LayerDetails.kind.

Why

tonkintaylor/yyyttnz-0730-6-009-cv-landslides (#13) needs AOI-clipped LINZ DEMs. We are hand-rolling a thin client there for now and intend to migrate to koop once the above land — koop is the right home for Koordinates operations and already has the version-aware caching, export/poll/download/unzip flow, and multi-domain auth.

Acceptance

• get_layer_from_id(..., extent=<geojson>) produces a server-side cropped export.
• Raster (grid) layers download and cache as .tif and are reused on repeat requests (version-aware, as today for vector).
• Existing vector behaviour unchanged.