feat(l1): intake dispatch via match_or_build + next-node + escalations endpoints

- /intake now runs match_or_build (matched/suggest/out_of_scope/build); build
  seeds the classified category as a hidden meta walked_path entry, matched starts
  a flow session, suggest/out_of_scope return prompt data with no session.
- New POST /sessions/{id}/next-node (threads node_text to advance_ai_build) and
  GET /escalations (engineer-or-above) for the handoff queue.
- New IntakeResponse(outcome=...)/NextNodeRequest/NextNodeResponse schemas and
  require_account_owner_or_admin dep.
- Reconcile Phase-1 intake tests to the new contract (mock match_or_build); add
  test_l1_api_ai_build.py covering build/out_of_scope/suggest/next-node/escalations.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app/schemas/l1.py

@@ -11,13 +11,31 @@ class IntakeRequest(BaseModel):
     customer_name: Optional[str] = None
     customer_contact: Optional[str] = None
     flow_id: Optional[UUID] = None
+    force_build: bool = False
 
 
 class IntakeResponse(BaseModel):
-    session_id: UUID
-    session_kind: Literal["flow", "proposal", "adhoc"]
-    ticket_id: str
-    ticket_kind: Literal["psa", "internal"]
+    outcome: Literal["matched", "suggest", "out_of_scope", "build"]
+    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
+    flow_id: Optional[UUID] = None   # for 'matched'
+    near_miss: Optional[dict] = None  # for 'suggest'
+    category: Optional[str] = None   # for 'out_of_scope'
+
+
+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
+    note: Optional[str] = None
+
+
+class NextNodeResponse(BaseModel):
+    node: dict
+    session_status: str
 
 
 class StepRequest(BaseModel):