feat(l1): l1_session_service start_flow/proposal/adhoc

Three start_* functions creating L1WalkSession rows with appropriate
session_kind and target id. Engineers acting in L1 mode get
acting_as='l1_coverage' for audit; native l1_tech users get acting_as=None.

step/notes (T13) and resolve/escalate (T14) extend this file next.

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

backend/app/services/l1_session_service.py

@@ -0,0 +1,93 @@
+"""L1 session lifecycle: start (flow/proposal/adhoc), step, notes, resolve, escalate.
+
+start_* functions live in T12; step/notes are T13; resolve/escalate are T14.
+"""
+from typing import Optional
+from uuid import UUID
+
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.models.l1_walk_session import L1WalkSession
+from app.models.user import User
+
+
+def _resolve_acting_as(user: User) -> Optional[str]:
+    """An engineer (whether covering or not) gets tagged for audit when using L1 surface.
+
+    Returns 'l1_coverage' for engineers (only engineers WITH the coverage flag should
+    reach this code path — the require_l1_or_coverage dep gates that). For native
+    l1_tech users, returns None (no special tag — they ARE l1).
+    """
+    if user.account_role == "engineer":
+        return "l1_coverage"
+    return None
+
+
+async def start_flow_session(
+    db: AsyncSession,
+    *,
+    account_id: UUID,
+    user: User,
+    flow_id: UUID,
+    ticket_id: str,
+    ticket_kind: str,  # 'psa' | 'internal'
+) -> L1WalkSession:
+    """Start a session walking an authored flow."""
+    session = L1WalkSession(
+        account_id=account_id,
+        created_by_user_id=user.id,
+        acting_as=_resolve_acting_as(user),
+        ticket_id=ticket_id,
+        ticket_kind=ticket_kind,
+        session_kind="flow",
+        flow_id=flow_id,
+    )
+    db.add(session)
+    await db.flush()
+    return session
+
+
+async def start_proposal_session(
+    db: AsyncSession,
+    *,
+    account_id: UUID,
+    user: User,
+    flow_proposal_id: UUID,
+    ticket_id: str,
+    ticket_kind: str,
+) -> L1WalkSession:
+    """Start a session walking an AI-built FlowProposal."""
+    session = L1WalkSession(
+        account_id=account_id,
+        created_by_user_id=user.id,
+        acting_as=_resolve_acting_as(user),
+        ticket_id=ticket_id,
+        ticket_kind=ticket_kind,
+        session_kind="proposal",
+        flow_proposal_id=flow_proposal_id,
+    )
+    db.add(session)
+    await db.flush()
+    return session
+
+
+async def start_adhoc_session(
+    db: AsyncSession,
+    *,
+    account_id: UUID,
+    user: User,
+    ticket_id: str,
+    ticket_kind: str,
+) -> L1WalkSession:
+    """Start an ad-hoc session with no tree (free-form note-taking only)."""
+    session = L1WalkSession(
+        account_id=account_id,
+        created_by_user_id=user.id,
+        acting_as=_resolve_acting_as(user),
+        ticket_id=ticket_id,
+        ticket_kind=ticket_kind,
+        session_kind="adhoc",
+    )
+    db.add(session)
+    await db.flush()
+    return session