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/schemas/l1.py

@@ -3,33 +3,54 @@ from datetime import datetime
 from typing import Any, Literal, Optional
 from uuid import UUID
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, Field, model_validator
 
 
 class IntakeRequest(BaseModel):
     problem_statement: str = Field(..., min_length=1)
     customer_name: Optional[str] = None
     customer_contact: Optional[str] = None
+    # When set, bypass the matcher and start this published flow directly (the
+    # suggest card's "Use this flow" — the client already holds the flow id).
     flow_id: Optional[UUID] = None
+    # When True, start an ad-hoc free-form walk (the out_of_scope prompt's
+    # "Walk it ad-hoc" fallback). Mutually informative with flow_id/force_build;
+    # flow_id takes precedence if both are somehow set.
+    adhoc: bool = False
     force_build: bool = False
 
 
+# Outcomes that start a session (and therefore must carry session_id + ticket).
+_SESSION_OUTCOMES = {"matched", "build", "adhoc"}
+
+
 class IntakeResponse(BaseModel):
-    outcome: Literal["matched", "suggest", "out_of_scope", "build"]
+    outcome: Literal["matched", "suggest", "out_of_scope", "build", "adhoc"]
     session_id: Optional[UUID] = None
     session_kind: Optional[Literal["flow", "proposal", "adhoc", "ai_build"]] = None
     ticket_id: Optional[str] = None
-    ticket_kind: Optional[str] = None
+    ticket_kind: Optional[Literal["psa", "internal"]] = None
     flow_id: Optional[UUID] = None   # for 'matched'
     near_miss: Optional[dict] = None  # for 'suggest'
     category: Optional[str] = None   # for 'out_of_scope'
 
+    @model_validator(mode="after")
+    def _check_outcome_invariants(self) -> "IntakeResponse":
+        """Restore the per-outcome contract the frontend depends on: a session
+        outcome MUST carry the session_id + ticket the walker navigates to, so a
+        backend regression surfaces here instead of as /l1/walk/undefined."""
+        if self.outcome in _SESSION_OUTCOMES:
+            if self.session_id is None or self.ticket_id is None:
+                raise ValueError(
+                    f"intake outcome '{self.outcome}' requires session_id + ticket_id"
+                )
+        return self
+
 
 class NextNodeRequest(BaseModel):
     node_id: Optional[str] = None
     node_text: Optional[str] = None  # rendered text of the node being answered (carry-forward Task 8)
-    answer: Optional[str] = None     # 'yes' | 'no' for questions
-    acknowledged: Optional[bool] = None
+    answer: Optional[str] = None     # 'yes' | 'no' for questions; None acks an instruction
     note: Optional[str] = None
 
 
@@ -70,6 +91,8 @@ class EscalateWithoutWalkRequest(BaseModel):
 class WalkSessionResponse(BaseModel):
     id: UUID
     session_kind: str
+    category: Optional[str] = None
+    problem_text: Optional[str] = None
     flow_id: Optional[UUID]
     flow_proposal_id: Optional[UUID]
     current_node_id: Optional[str]