feat(modules): group-files schema + owner-relative path helpers + repo (§11.6 files C1)

Extends the §11.6 shared-collection machinery to the filesystem-backed
`files` blob store (after KV/0053 and docs/0054).

- 0055_group_files.sql: widen the group_collections kind CHECK to admit
  'files'; add a group-keyed `group_files` metadata table mirroring
  `files` (0018), CASCADE on group delete.
- files_repo: generalize the four path/IO free functions
  (shard_dir_at / final_path_at / write_atomic_at / read_verify_at) to
  take an owner-relative dir instead of `app_id`, and make them (plus the
  cursor helpers) pub(crate). The security-sensitive atomic-write +
  checksum-on-read mechanics now have a single source shared by the app
  and group repos. App behavior is unchanged (apps shard at `<app_id>/`).
- group_files_repo: FsGroupFilesRepo keyed by group_id, blobs at
  `<root>/files/groups/<group_id>/<collection>/<id[0:2]>/<id>` — a
  `groups/` infix disjoint from the per-app subtree, so the existing
  recursive orphan sweeper covers it with zero change.

Schema snapshot blessed (55 migrations); app-files fs tests still green.

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

crates/manager-core/tests/expected_schema.txt

@@ -239,6 +239,17 @@ table: group_docs
   created_at: timestamp with time zone NOT NULL default=now()
   updated_at: timestamp with time zone NOT NULL default=now()
 
+table: group_files
+  group_id: uuid NOT NULL
+  collection: text NOT NULL
+  id: uuid NOT NULL
+  name: text NOT NULL
+  content_type: text NOT NULL
+  size_bytes: bigint NOT NULL
+  checksum_sha256: text NOT NULL
+  created_at: timestamp with time zone NOT NULL default=now()
+  updated_at: timestamp with time zone NOT NULL default=now()
+
 table: group_kv_entries
   group_id: uuid NOT NULL
   collection: text NOT NULL
@@ -522,6 +533,10 @@ indexes on group_docs:
   idx_group_docs_data_gin: public.group_docs USING gin (data jsonb_path_ops)
   idx_group_docs_group_collection: public.group_docs USING btree (group_id, collection)
 
+indexes on group_files:
+  group_files_pkey: public.group_files USING btree (group_id, collection, id)
+  idx_group_files_group_collection: public.group_files USING btree (group_id, collection)
+
 indexes on group_kv_entries:
   group_kv_entries_pkey: public.group_kv_entries USING btree (group_id, collection, key)
   idx_group_kv_entries_group_collection: public.group_kv_entries USING btree (group_id, collection)
@@ -727,7 +742,7 @@ constraints on files_trigger_details:
   [PRIMARY KEY] files_trigger_details_pkey: PRIMARY KEY (trigger_id)
 
 constraints on group_collections:
-  [CHECK] group_collections_kind_check: CHECK ((kind = ANY (ARRAY['kv'::text, 'docs'::text])))
+  [CHECK] group_collections_kind_check: CHECK ((kind = ANY (ARRAY['kv'::text, 'docs'::text, 'files'::text])))
   [CHECK] group_collections_owner_exactly_one: CHECK (((group_id IS NULL) <> (app_id IS NULL)))
   [FOREIGN KEY] group_collections_app_id_fkey: FOREIGN KEY (app_id) REFERENCES apps(id) ON DELETE CASCADE
   [FOREIGN KEY] group_collections_group_id_fkey: FOREIGN KEY (group_id) REFERENCES groups(id) ON DELETE CASCADE
@@ -737,6 +752,10 @@ constraints on group_docs:
   [FOREIGN KEY] group_docs_group_id_fkey: FOREIGN KEY (group_id) REFERENCES groups(id) ON DELETE CASCADE
   [PRIMARY KEY] group_docs_pkey: PRIMARY KEY (group_id, collection, id)
 
+constraints on group_files:
+  [FOREIGN KEY] group_files_group_id_fkey: FOREIGN KEY (group_id) REFERENCES groups(id) ON DELETE CASCADE
+  [PRIMARY KEY] group_files_pkey: PRIMARY KEY (group_id, collection, id)
+
 constraints on group_kv_entries:
   [FOREIGN KEY] group_kv_entries_group_id_fkey: FOREIGN KEY (group_id) REFERENCES groups(id) ON DELETE CASCADE
   [PRIMARY KEY] group_kv_entries_pkey: PRIMARY KEY (group_id, collection, key)
@@ -881,3 +900,4 @@ constraints on vars:
   0052: group collections
   0053: group kv entries
   0054: group docs
+  0055: group files