feat: serve each building's usage class from its source function code #210

Problem

GET /api/v1/buildings returns function_code exactly as the source dataset stores it: the ALKIS Gebäudefunktion in Germany (for example 31001_1000), null for the Netherlands, Austria and Czechia. Every consumer then needs its own per-country code table to know what a building is used for.

Each data provider describes use differently: a different code list, or a different attribute altogether. In EnerPlanET a 3D building with no OSM outline currently cannot be classified and is treated as residential, for example DEBY_LOD2_7672144 (about 5,500 m², THD campus).

The knowledge of what a source code means belongs next to the source data, in one tool, so that adding a region is a change here only.

Proposal

• A mapping per dataset from the source use attribute and its codes to a usage class, kept with the country or region configuration. For Germany it follows the official ALKIS Gebäudefunktion code list.
• Applied in the pipeline step that extracts function_code; the result is stored with the building.
• Served on GET /api/v1/buildings next to function_code:
  • use_type: a neutral use category (for example residential, office, education, retail, industry, agriculture, other);
  • residential: boolean;
  • both null when the code is missing or unmapped.
• Adding a region means adding its mapping here; consumers need no change.

To agree before building

• The use_type vocabulary. Consumers map it to their own categories (EnerPlanET maps it to pylovo's building classes), so it stays neutral and is not tied to any one consumer.
• Where the code lists live (a file per country in the configuration, or a table in the database) and the source and licence of each list.
• How mixed-use codes are classified.

Done when

• German buildings carry use_type and residential derived from their ALKIS code.
• Netherlands, Austria and Czechia return null until a mapping is added.
• OpenAPI description and changelog updated; an integration test covers known German codes.