feat(hierarchies): multi-repo single-owner ownership + structural prune (M3)

§7 of the groups/project-tool design. A group node is now authoritatively
managed by exactly one project-root; `pic apply --dir` claims, conflicts,
takes over, and (with --prune) structurally reaps owned nodes.

- Migration 0052: `projects(id, key UNIQUE)` + promote the inert
  `groups.owner_project` (0047) to a real FK (ON DELETE SET NULL) + index.
- CLI mints a stable, gitignored project key in `.picloud/project.json`
  (`pic init`, or lazily on first tree plan/apply) and presents it on every
  tree request; `pic apply --dir --takeover` flag.
- Server: prepare_tree resolves ownership read-only (plan surfaces conflicts
  + prune candidates; token folds each group's owner key). apply_tree upserts
  the project in-tx, claims created groups on insert, reconciles existing-group
  ownership under the per-node advisory lock (first-commit-wins), and prunes
  owned-but-undeclared groups leaf-first (delete=RESTRICT, never another repo's
  or a UI-owned node).
- Authz (§7.4, ownership ⟂ RBAC): takeover requires GroupAdmin per contested
  node — enforced in authz_tree (pre-tx) AND re-verified in-tx at the ownership
  decision, so --force (which waives the staleness token) can't open a
  takeover-without-admin window. The attacker-supplied project key is
  length/charset-validated server-side.
- Tests: tests/ownership.rs (claim → conflict → takeover → flip; non-admin
  takeover → 403; prune-owned-only); format_conflicts unit test; schema golden
  reblessed. tree_shape M2 journey updated to reparent in-place (a fresh dir is
  now a distinct project and would correctly conflict).

Closes the M3 milestone; reviewed (4 findings: 1 authz-bypass via --force +
key validation + 2 LOW, all fixed). M4 (trigger/route templates) remains.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/picloud-cli/tests/common/member.rs

@@ -78,6 +78,26 @@ pub fn grant_membership(fx: &Fixture, app_slug: &str, user_id: &str, role: &str)
     );
 }
 
+/// `POST /api/v1/admin/groups/{slug}/members` — grant `role` on a group.
+pub fn grant_group_membership(fx: &Fixture, group_slug: &str, user_id: &str, role: &str) {
+    let client = reqwest::blocking::Client::new();
+    let resp = client
+        .post(format!(
+            "{}/api/v1/admin/groups/{}/members",
+            fx.url, group_slug
+        ))
+        .bearer_auth(&fx.admin_token)
+        .json(&json!({ "user_id": user_id, "role": role }))
+        .send()
+        .expect("grant group membership");
+    assert!(
+        resp.status().is_success(),
+        "grant group membership failed: {} {}",
+        resp.status(),
+        resp.text().unwrap_or_default(),
+    );
+}
+
 /// `PATCH /api/v1/admin/apps/{slug}/members/{user_id}` — promote/demote.
 pub fn update_membership(fx: &Fixture, app_slug: &str, user_id: &str, role: &str) {
     let client = reqwest::blocking::Client::new();