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/fork_point.py

@@ -23,6 +23,12 @@ class ForkPoint(Base):
 
     id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
     session_id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), ForeignKey("ai_sessions.id", ondelete="CASCADE"), nullable=False, index=True)
+    account_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("accounts.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
     parent_branch_id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), ForeignKey("session_branches.id", ondelete="CASCADE"), nullable=False)
     trigger_step_id: Mapped[Optional[uuid.UUID]] = mapped_column(UUID(as_uuid=True), ForeignKey("ai_session_steps.id", ondelete="SET NULL"), nullable=True)
     fork_reason: Mapped[str] = mapped_column(Text, nullable=False)