fix(l1): resolve PR #193 backend review findings (1,4,5,6,7,8,9,10)

Server-assigns a uuid4 id to every AI-generated node (Finding 1 showstopper:
nodes had no id but the advance protocol keys on node_id, so ai_build walks
never advanced past question 1). Replaces the hidden {"node_type":"meta"}
walked_path convention with real category/problem_text/pending_node columns on
l1_walk_sessions (migration 61dda4f615c6) — fixes junk proposals + off-by-one
depth cap (Findings 8,9), and pending_node replays the served node on re-mount
(no duplicate paid LLM call). Intake honors explicit flow_id and adhoc=True
(Findings 4,5); flow_proposals.l1_session_id FK -> CASCADE (Finding 6 time
bomb); L1 category GET is owner+admin like PATCH and require_account_owner_or_admin
delegates to User.can_manage_account (Finding 7); escalate falls back to default
recipients + filters deleted_at + warns when empty (Finding 10). Cleanups: dead
ticket_ref removed, IntakeResponse per-outcome validator, unused acknowledged
dropped, escalations partial index, restored a deleted audit assertion.

Full Phase 2A backend set: 110 passed / 0 failed.

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

backend/app/api/deps.py

@@ -279,10 +279,11 @@ async def require_account_owner(
 async def require_account_owner_or_admin(
     current_user: Annotated[User, Depends(get_current_active_user)]
 ) -> User:
-    """Require account owner or account-admin (blocks engineers); super_admin bypass."""
-    if current_user.is_super_admin:
-        return current_user
-    if current_user.account_role in ("owner", "admin"):
+    """Require account owner or account-admin (blocks engineers); super_admin bypass.
+
+    Delegates to ``User.can_manage_account`` so the rule lives in exactly one place.
+    """
+    if current_user.can_manage_account:
         return current_user
     raise HTTPException(
         status_code=status.HTTP_403_FORBIDDEN,