feat(l1): l1_session_service record_step + update_notes

record_step appends to walked_path JSONB and advances current_node_id
on flow/proposal walks; refuses adhoc sessions. update_notes replaces
walk_notes (used by adhoc walks for debounced autosave); 256KB size cap
to prevent unbounded JSONB growth. Both reject non-active sessions.

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

backend/app/services/l1_session_service.py

@@ -2,6 +2,8 @@
 
 start_* functions live in T12; step/notes are T13; resolve/escalate are T14.
 """
+import json
+from datetime import datetime, timezone
 from typing import Optional
 from uuid import UUID
 
@@ -91,3 +93,59 @@ async def start_adhoc_session(
     db.add(session)
     await db.flush()
     return session
+
+
+async def record_step(
+    db: AsyncSession,
+    *,
+    session_id: UUID,
+    node_id: str,
+    question: str,
+    answer: str,
+    note: Optional[str] = None,
+) -> L1WalkSession:
+    """Record an answered step in a tree walk. Appends to walked_path JSONB and
+    advances current_node_id. Raises ValueError on adhoc sessions or inactive
+    sessions. Updates last_step_at."""
+    session = await db.get(L1WalkSession, session_id)
+    if not session:
+        raise ValueError(f"L1WalkSession {session_id} not found")
+    if session.session_kind == "adhoc":
+        raise ValueError("Cannot record step on adhoc session — use update_notes")
+    if session.status != "active":
+        raise ValueError(f"Session {session_id} is not active (status={session.status})")
+    entry = {
+        "node_id": node_id,
+        "question": question,
+        "answer": answer,
+        "l1_note": note,
+    }
+    # JSONB requires assigning a new list — in-place mutation isn't tracked
+    session.walked_path = [*session.walked_path, entry]
+    session.current_node_id = node_id
+    session.last_step_at = datetime.now(timezone.utc)
+    await db.flush()
+    return session
+
+
+async def update_notes(
+    db: AsyncSession,
+    *,
+    session_id: UUID,
+    notes: list[dict],
+) -> L1WalkSession:
+    """Replace walk_notes on an active session. Used by adhoc walks for
+    debounced autosave. Raises ValueError if missing or inactive. Caps notes
+    payload at 256KB to prevent unbounded growth."""
+    session = await db.get(L1WalkSession, session_id)
+    if not session:
+        raise ValueError(f"L1WalkSession {session_id} not found")
+    if session.status != "active":
+        raise ValueError(f"Session {session_id} is not active (status={session.status})")
+    encoded_size = len(json.dumps(notes).encode("utf-8"))
+    if encoded_size > 256 * 1024:
+        raise ValueError("walk_notes exceeds 256KB cap — consider escalating")
+    session.walk_notes = notes
+    session.last_step_at = datetime.now(timezone.utc)
+    await db.flush()
+    return session