Slurper: House of Graphs invariants

[katjabercic]

Extend source coverage to House of Graphs: just the invariants (e.g. "chromatic number", "girth", "treewidth"), not the graphs or invariant values.

Source

• Endpoint: GET https://houseofgraphs.org/api/invariants — do not send
  Accept: application/json (it returns {"error":true,"message":"Unauthorized"});
  the default Accept: */* returns HAL+JSON. No auth, no pagination.
• Shape: _embedded.invariantModelList[].entity → invariantId,
  invariantName (= keyword), definition (HTML), typeName (b/i/r).
  ~52 entries.

Tasks

• web/slurper/source_house_of_graphs.py with a slurper class + module-level
  instance.
• Add a HOUSE_OF_GRAPHS choice to Item.Source.
• Map each invariant → Item(source=HOUSE_OF_GRAPHS, identifier=str(invariantId), url=f"https://houseofgraphs.org/invariants/{id}", name=invariantName, description=<definition with HTML stripped>).
• New Item.Source member with a ≤4-char tag (e.g. HoG = "HoG"); add it to
  Item.Source.key().
• management/commands/import_house_of_graphs.py and clear_house_of_graphs.py.
• Wire the importer into populate-db.
• Makefile target import-house-of-graphs.
• make create-migrations for the new Source choice.
• Make basic tests.
• make fix-files clean; tests pass.

Notes

• No Wikidata cross-link property for invariants — they connect to existing concepts
  via the NAME_EQ heuristic (Chromatic Number, Girth, …), which makes them handy
  for the alignment-benchmark work.