feat(l1): L1 endpoint surface (intake/queue/step/notes/resolve/escalate)

Mounts /api/v1/l1/* with require_l1_or_coverage on every route. Intake
creates an internal ticket and starts a flow OR adhoc session (PSA queue
merge follows in Phase 2). Step/notes/resolve/escalate delegate to
l1_session_service. escalate-without-walk creates an immediately-
escalated session for the BuildAbortedNoKB path.

ValueError from services → 400. Cross-account session access → 404.

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

backend/app/schemas/l1.py

@@ -0,0 +1,72 @@
+"""Pydantic schemas for the /l1/* endpoint surface."""
+from datetime import datetime
+from typing import Any, Literal, Optional
+from uuid import UUID
+
+from pydantic import BaseModel, Field
+
+
+class IntakeRequest(BaseModel):
+    problem_statement: str = Field(..., min_length=1)
+    customer_name: Optional[str] = None
+    customer_contact: Optional[str] = None
+    flow_id: Optional[UUID] = None
+
+
+class IntakeResponse(BaseModel):
+    session_id: UUID
+    session_kind: Literal["flow", "proposal", "adhoc"]
+    ticket_id: str
+    ticket_kind: Literal["psa", "internal"]
+
+
+class StepRequest(BaseModel):
+    node_id: str
+    question: str
+    answer: str
+    note: Optional[str] = None
+
+
+class NotesRequest(BaseModel):
+    notes: list[dict[str, Any]]
+
+
+class ResolveRequest(BaseModel):
+    helpful: bool
+    resolution_notes: str
+
+
+class EscalateRequest(BaseModel):
+    reason: Optional[str] = None
+    reason_category: str = Field(..., min_length=1)
+
+
+class EscalateWithoutWalkRequest(BaseModel):
+    problem_statement: str = Field(..., min_length=1)
+    customer_name: Optional[str] = None
+    customer_contact: Optional[str] = None
+    reason_category: str = Field(..., min_length=1)
+    reason: Optional[str] = None
+
+
+class WalkSessionResponse(BaseModel):
+    id: UUID
+    session_kind: str
+    flow_id: Optional[UUID]
+    flow_proposal_id: Optional[UUID]
+    current_node_id: Optional[str]
+    walked_path: list[dict[str, Any]]
+    walk_notes: list[dict[str, Any]]
+    status: str
+    started_at: datetime
+    last_step_at: datetime
+    resolved_at: Optional[datetime]
+
+
+class QueueRow(BaseModel):
+    ticket_id: str
+    ticket_kind: Literal["psa", "internal"]
+    problem_statement: Optional[str] = None
+    customer_name: Optional[str] = None
+    status: str
+    created_at: Optional[datetime] = None