v0.48.0 — SLICE-2: collection field schema + central validation (§22.4a)

Collections declare a `fields:` schema in `.collection.yaml`; entries carry
typed metadata (enum/tags/text). New central `metadata_schema` module parses
the schema leniently (INV-3) and validates entry values — advisory at read
(corpus mirror flags violations as `metadata_malformed` without hard-failing),
the enforcement point for the write boundary (edit endpoints land SLICE-4/5).

- app/metadata_schema.py: parse_fields (lenient/normalizing) + validate
- registry.parse_collection_manifest reads `fields:` into collection config
- collections.get_collection unpacks `fields`; served by the collection API
- cache._refresh_collection_corpus validates each entry advisory-only

Non-breaking, opt-in: no `fields:` → unchanged (INV-5); default `document`
collection declares none (N=1 unchanged). No DB migration — schema rides in
collections.config_json. SLICE-2 of
docs/design/2026-06-06-configurable-collection-metadata.md §7.2.

Backend suite green (601 passed).

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

backend/app/collections.py

@@ -45,6 +45,21 @@ def _enabled_models_from_config(config_json: str | None) -> list[str] | None:
     return [str(m) for m in em] if isinstance(em, list) else None
 
 
+def _fields_from_config(config_json: str | None) -> dict | None:
+    """§22.4a SLICE-2 per-collection metadata field schema from a `config_json`
+    blob, or None when the collection declares no `fields:`. The stored value is
+    already normalized by `metadata_schema.parse_fields` at ingest, so it's
+    served verbatim."""
+    if not config_json:
+        return None
+    try:
+        cfg = json.loads(config_json)
+    except (json.JSONDecodeError, TypeError):
+        return None
+    fields = cfg.get("fields") if isinstance(cfg, dict) else None
+    return fields if isinstance(fields, dict) and fields else None
+
+
 def default_collection_id(project_id: str) -> str:
     """The id of a project's default (S1: sole) collection. Falls back to the
     literal 'default' when the project has no collection row yet."""
@@ -102,7 +117,12 @@ def get_collection(collection_id: str) -> dict | None:
     if row is None:
         return None
     out = dict(row)
-    out["enabled_models"] = _enabled_models_from_config(out.pop("config_json", None))
+    config_json = out.pop("config_json", None)
+    out["enabled_models"] = _enabled_models_from_config(config_json)
+    # §22.4a SLICE-2: serve the collection's metadata field schema (None when
+    # the collection declares no `fields:` — INV-5, the default `document`
+    # collection is unaffected).
+    out["fields"] = _fields_from_config(config_json)
     out["entry_noun"] = entry_noun(out["type"])
     return out