feat: add branching columns to ai_sessions, ai_session_steps, file_uploads

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/models/ai_session_step.py

@@ -16,6 +16,8 @@ from app.core.database import Base
 if TYPE_CHECKING:
     from app.models.ai_session import AISession
     from app.models.script_template import ScriptGeneration
+    from app.models.session_branch import SessionBranch
+    from app.models.fork_point import ForkPoint
 
 
 class AISessionStep(Base):
@@ -34,7 +36,7 @@ class AISessionStep(Base):
     __table_args__ = (
         CheckConstraint(
             "step_type IN ('question', 'action', 'script_generation', 'verification', "
-            "'info_request', 'note', 'intake_analysis')",
+            "'info_request', 'note', 'intake_analysis', 'fork')",
             name="ck_ai_session_steps_step_type",
         ),
     )
@@ -119,6 +121,24 @@ class AISessionStep(Base):
         Integer, nullable=False, default=0,
     )
 
+    # ── Branching ──
+    branch_id: Mapped[Optional[uuid.UUID]] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("session_branches.id", ondelete="SET NULL"),
+        nullable=True,
+        index=True,
+        comment="NULL = pre-branching/root messages",
+    )
+    is_fork_point: Mapped[bool] = mapped_column(
+        default=False,
+        comment="Whether this step triggered a fork",
+    )
+    fork_point_id: Mapped[Optional[uuid.UUID]] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("fork_points.id", ondelete="SET NULL"),
+        nullable=True,
+    )
+
     # ── Timestamps ──
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)