Add governed VMM flood hazard scenarios

docs/DATABASE_IMPLEMENTATION_PLAN.md

@@ -269,6 +269,19 @@ public provider endpoint's `not_configured` status.
 - User accounts.
 - Full model registry tables.
 
+## Governed flood-hazard rasters
+
+VMM flood-depth scenarios require no new table. Each acquired coverage is an
+ordinary `datasets` raster plus immutable `dataset_versions` provenance and a
+normal synchronous acquisition Job. The GeoTIFF remains filesystem/object
+storage; PostgreSQL keeps source identity, WCS checksums, EPSG:31370 bounds,
+source/normalized units, exact Area scope and scenario parameters.
+
+Scenario alternatives do not receive a fabricated `observed_at` value and are
+not grouped as a temporal series. Selection metrics are calculated on demand
+from the persisted raster. Bathymetry and permanent waterbody volume remain
+absent from persistence until a separately governed source/model exists.
+
 ## Temporal dataset foundation
 
 Historical observations remain normal `datasets` and `vector_features`; there