feat: Phase 1 Group 2 — add account_id to AI branching tables

Tables: session_branches, session_handoffs, fork_points,
        ai_session_steps, ai_suggestions
Backfill: session_id → ai_sessions.account_id (all except
ai_suggestions which uses user_id → users.account_id)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/models/ai_session_step.py

@@ -50,6 +50,13 @@ class AISessionStep(Base):
         nullable=False,
         index=True,
     )
+    account_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("accounts.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+        comment="Denormalized from ai_sessions.account_id for direct tenant filtering.",
+    )
     step_order: Mapped[int] = mapped_column(
         Integer, nullable=False,
         comment="Sequential position in the session (0-indexed)",