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/migrations/0055_group_files.sql

@@ -0,0 +1,37 @@
+-- §11.6 (cont., v1.2 Hierarchies): group-shared FILES collections.
+--
+-- Extends the shared-collection machinery (0052 registry + the per-kind stores
+-- 0053 group_kv_entries / 0054 group_docs) from KV/docs to the filesystem-backed
+-- `files` blob store. The registry's `kind` discriminator was built for exactly
+-- this — widen its CHECK to admit 'files', and add a group-keyed metadata table
+-- mirroring `files` (0018).
+
+-- Widen the marker kind allow-list (auto-named `group_collections_kind_check`,
+-- per 0052's inline column CHECK; 0054 widened it to ('kv','docs')).
+ALTER TABLE group_collections
+    DROP CONSTRAINT group_collections_kind_check,
+    ADD CONSTRAINT group_collections_kind_check CHECK (kind IN ('kv', 'docs', 'files'));
+
+-- Group-keyed file metadata. Identity tuple `(group_id, collection, id)` — keyed
+-- by the OWNING GROUP, not an app (the shared blobs belong to the group). The
+-- bytes live on disk at
+--   <PICLOUD_FILES_ROOT>/files/groups/<group_id>/<collection>/<id[0:2]>/<id>
+-- (a `groups/` infix disjoint from the per-app `files/<app_id>/...` subtree, so
+-- the orphan sweeper covers both with one walk). CASCADE on group delete (the
+-- metadata dies with its group; an app delete leaves it), matching 0053/0054.
+CREATE TABLE group_files (
+    group_id        UUID NOT NULL REFERENCES groups(id) ON DELETE CASCADE,
+    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,           -- hex, 64 chars, lowercase
+    created_at      TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at      TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    PRIMARY KEY (group_id, collection, id)
+);
+
+-- List + cursor pagination scans by (group_id, collection) — mirrors
+-- idx_files_app_collection (0018).
+CREATE INDEX idx_group_files_group_collection ON group_files (group_id, collection);