feat(groups): schema + root-group backfill (migration 0047)

Phase 2 (blueprint §5): groups form a single-parent org tree ABOVE apps.
This migration adds the tree + membership tables and gives every app a
parent (§9 adoption):

- groups: id, parent_id (self-FK ON DELETE RESTRICT), slug (instance-
  global UNIQUE, frozen at creation), name, description, structure_version
  (bumped on structural mutation), owner_project (inert §7 seam).
- group_members: (group_id, user_id, role) with the SAME three role
  literals as app_members so AppRole round-trips and the authz rank table
  covers both.
- apps.group_id: nullable add → backfill every app under a seeded 'root'
  group → promote to NOT NULL + FK RESTRICT.

No group-owned resources yet (scripts/secrets stay app-owned — Phase 3).

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

crates/manager-core/migrations/0047_groups.sql

@@ -0,0 +1,82 @@
+-- Phase 2: groups as a pure org / RBAC / UI container — see
+-- docs/design/groups-and-project-tool.md §5, §9.
+--
+-- Groups form a GitLab-like, single-parent tree ABOVE apps. Phase 2 adds
+-- the tree, hierarchy-aware membership, and structural-mutation safety —
+-- but NO group-owned resources yet (scripts/vars/secrets stay app-owned;
+-- that is Phase 3). The only data-plane touch is apps gaining a parent
+-- pointer.
+--
+-- Adoption (§9): every existing app must have a parent from day one so
+-- resolution always terminates. This migration seeds a single instance
+-- root group and reparents every app under it, then promotes
+-- apps.group_id to NOT NULL.
+
+CREATE TABLE groups (
+    id                UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    -- Single parent keeps inheritance acyclic and resolution
+    -- deterministic. NULL parent = a root node. RESTRICT (never CASCADE):
+    -- deleting a non-empty group is refused so descendant apps and their
+    -- isolated data can't be destroyed implicitly (§5.6). The ancestor-walk
+    -- cycle guard that keeps this acyclic lives in manager-core (a SQL
+    -- CHECK can't express it).
+    parent_id         UUID REFERENCES groups(id) ON DELETE RESTRICT,
+    -- Instance-global identifier, frozen at creation. A rename/reparent
+    -- updates the display name/path but NEVER rewrites the slug, so the
+    -- deployment key stays stable and external references don't break.
+    -- Format validation lives in Rust handlers (same rule as app slugs).
+    slug              TEXT NOT NULL UNIQUE,
+    name              TEXT NOT NULL,
+    description       TEXT,
+    -- Per-subtree structure version (§6): bumped on every structural
+    -- mutation of this node (reparent/rename/delete) so a future CLI/
+    -- orchestrator can detect structural drift. NOT an authz input —
+    -- authorization is resolved live every request.
+    structure_version BIGINT NOT NULL DEFAULT 1,
+    -- §7 ownership seam — the project-root that manages this node. Inert
+    -- in Phase 2 (no projects table yet); nullable, no FK.
+    owner_project     UUID,
+    created_at        TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at        TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX groups_parent_id_idx ON groups (parent_id);
+
+-- Per-(user, group) explicit grant, mirroring app_members. Inherited
+-- membership (GitLab-style) is resolved in code by walking ancestors: a
+-- group_admin on any ancestor is implicitly app_admin on every app and
+-- subgroup beneath it. Roles reuse the SAME three literals as app_members
+-- so AppRole round-trips with zero mapping and the authz rank table
+-- covers both tables.
+CREATE TABLE group_members (
+    group_id    UUID NOT NULL REFERENCES groups(id) ON DELETE CASCADE,
+    user_id     UUID NOT NULL REFERENCES admin_users(id) ON DELETE CASCADE,
+    role        TEXT NOT NULL CHECK (role IN ('app_admin', 'editor', 'viewer')),
+    created_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    PRIMARY KEY (group_id, user_id)
+);
+
+-- Hot path is the authz ancestor walk "what groups does this user have a
+-- role on?" plus the per-group member list.
+CREATE INDEX group_members_user_id_idx ON group_members (user_id);
+
+-- Add the parent pointer to apps (nullable for the backfill window).
+ALTER TABLE apps ADD COLUMN group_id UUID;
+
+-- Seed a single instance root group and reparent every existing app under
+-- it. App slugs are already instance-global, so no slug rewrite is needed
+-- — the parent pointer is new metadata layered on top.
+WITH root_group AS (
+    INSERT INTO groups (slug, name, description)
+    VALUES ('root', 'Root', 'The instance root group — parent of all apps created before groups landed.')
+    RETURNING id
+)
+UPDATE apps SET group_id = (SELECT id FROM root_group);
+
+-- Every app now has a parent; promote to NOT NULL + FK. RESTRICT so a
+-- group with apps can't be deleted out from under them (§5.6).
+ALTER TABLE apps ALTER COLUMN group_id SET NOT NULL;
+ALTER TABLE apps
+    ADD CONSTRAINT apps_group_id_fk FOREIGN KEY (group_id) REFERENCES groups(id) ON DELETE RESTRICT;
+
+CREATE INDEX apps_group_id_idx ON apps (group_id);