§22 S6: per-collection enabled_models — the §22.12 model-universe chain

A collection's .collection.yaml may carry an enabled_models list that NARROWS
its project's universe, which narrows the deployment ENABLED_MODELS. Resolution
(extending §6.6/§6.7): funder ∩ per-entry models ∩ collection ∩ project, with
the operator providers as the ceiling.

- migration 031: additive collections.config_json (parallels projects.config_json).
- registry.parse_collection_manifest: read enabled_models into CollectionEntry.config
  (absent = inherit, present incl. [] = narrow; [] opts the collection out of AI);
  reject a non-list. _upsert_named_collection persists config_json. The default
  collection (from projects.yaml) leaves it NULL — it inherits the project.
- models_resolver: _scope_narrowed_universe narrows the operator universe by the
  entry's project then collection enabled_models; the funder universe is bounded
  by it too. A collection cannot widen its project (narrowing from the ceiling).
- collections.get_collection surfaces enabled_models; GET
  /api/projects/:id/collections/:cid returns it.
- test_s6_collection_models_vertical.py: 9 cases (parse, the narrowing chain
  incl. opt-out + cannot-widen, API surfacing).

Backend: 525 passed. Per SPEC §22.12. Releases as part of v0.45.0.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/migrations/031_collection_enabled_models.sql

@@ -0,0 +1,9 @@
+-- §22.12 S6 — per-collection model universe.
+--
+-- A collection's `.collection.yaml` may carry an `enabled_models` list that
+-- NARROWS its project's universe (which narrows the deployment ENABLED_MODELS).
+-- Mirrored into a `config_json` blob on the collection row, paralleling
+-- `projects.config_json` (which already holds the project's enabled_models +
+-- theme). Additive only — no rebuild. NULL means "no per-collection narrowing;
+-- inherit the project's universe."
+ALTER TABLE collections ADD COLUMN config_json TEXT;