fix(l1): add index=True to L1WalkSession.last_step_at model column

Aligns the model with the migration (T6 review caught: migration creates
ix_l1_walk_sessions_last_step_at but model annotation was missing, causing
schema drift if Base.metadata.create_all is used in tests).

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

backend/app/models/l1_walk_session.py

@@ -128,6 +128,7 @@ class L1WalkSession(Base):
         DateTime(timezone=True),
         default=lambda: datetime.now(timezone.utc),
         onupdate=lambda: datetime.now(timezone.utc),
+        index=True,
     )
     resolved_at: Mapped[Optional[datetime]] = mapped_column(
         DateTime(timezone=True), nullable=True,