feat(slice4): POST .../meta single-entry edit endpoint + GET RFC meta/can_edit_meta (§22.4a PUC-1)

Direct-commit to the sidecar (D7), contributor+ gated (INV-4), schema-validated
at the write boundary, lazy-migrates a legacy entry, re-ingests. GET RFC now
returns the per-entry meta mapping and a can_edit_meta capability.

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

backend/app/api.py

@@ -29,6 +29,7 @@ from . import (
     api_invitations,
     api_join_requests,
     api_memberships,
+    api_metadata,
     api_notifications,
     api_prs,
     auth,
@@ -130,6 +131,8 @@ def make_router(
     router.include_router(api_prs.make_router(config, gitea, bot, providers))
     # Slice 5: §13 graduation + §13.1 claim.
     router.include_router(api_graduation.make_router(config, gitea, bot))
+    # §22.4a SLICE-4/5: entry metadata edit + Owner-gated collection migrate.
+    router.include_router(api_metadata.make_router(config, gitea, bot))
     # Slice 6: §15 notifications surface (inbox, watches, prefs,
     # quiet hours, per-user mute, email unsubscribe, bounce webhook).
     router.include_router(api_notifications.make_router(config))
@@ -721,6 +724,9 @@ def make_router(
             (slug,),
         ).fetchone()
         payload["proposed_use_case"] = uc["use_case"] if uc else None
+        # §22.4a SLICE-4: contributor+ on the entry's collection may edit metadata.
+        payload["can_edit_meta"] = bool(
+            auth.can_contribute_in_collection(viewer, auth.collection_of_rfc(slug)))
         return payload
 
     # ---------------------------------------------------------------
@@ -839,6 +845,9 @@ def make_router(
             (slug, collection_id),
         ).fetchone()
         payload["proposed_use_case"] = uc["use_case"] if uc else None
+        # §22.4a SLICE-4: contributor+ on the collection may edit metadata (INV-4).
+        payload["can_edit_meta"] = bool(
+            auth.can_contribute_in_collection(viewer, collection_id))
         return payload
 
     @router.get("/api/projects/{project_id}/rfcs")
@@ -1387,6 +1396,9 @@ def _serialize_rfc(row) -> dict[str, Any]:
         "tags": json.loads(row["tags_json"] or "[]"),
         "body": row["body"] or "",
         "metadata_malformed": bool(row["metadata_malformed"]),
+        # §22.4a SLICE-4: the full per-entry metadata mapping (known + custom
+        # fields) so the detail panel can render schema-driven controls.
+        "meta": json.loads(row["meta_json"] or "{}"),
     }