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/tests/test_collection_helpers.py

@@ -2,6 +2,7 @@
 subfolder_of."""
 from __future__ import annotations
 
+import json
 import tempfile
 from pathlib import Path
 
@@ -62,3 +63,23 @@ def test_get_collection_and_subfolder():
     assert collections_mod.subfolder_of("features") == "features"
     assert collections_mod.subfolder_of("default") == ""
     assert collections_mod.get_collection("nope") is None
+
+
+# ---- §22.4a SLICE-2: field schema served on the collection ----
+
+def test_get_collection_fields_none_when_unset():
+    # INV-5: a collection with no `fields:` exposes fields=None (the default
+    # `document` collection sees zero change).
+    _db()
+    _seed()
+    assert collections_mod.get_collection("features")["fields"] is None
+
+
+def test_get_collection_exposes_field_schema():
+    _db()
+    _seed()
+    schema = {"priority": {"type": "enum", "values": ["P0", "P1"]}}
+    db.conn().execute(
+        "UPDATE collections SET config_json = ? WHERE id = 'features'",
+        (json.dumps({"fields": schema}),))
+    assert collections_mod.get_collection("features")["fields"] == schema