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.py

@@ -20,6 +20,10 @@ if TYPE_CHECKING:
     from app.models.account import Account
     from app.models.tree import Tree
     from app.models.psa_connection import PsaConnection
+    from app.models.session_branch import SessionBranch
+    from app.models.fork_point import ForkPoint
+    from app.models.session_handoff import SessionHandoff
+    from app.models.session_resolution_output import SessionResolutionOutput
 
 
 class AISession(Base):
@@ -206,6 +210,28 @@ class AISession(Base):
         comment="Full LLM message history for context continuity",
     )
 
+    # ── Branching ──
+    is_branching: Mapped[bool] = mapped_column(
+        default=False,
+        comment="Whether conversational branching is active for this session",
+    )
+    active_branch_id: Mapped[Optional[uuid.UUID]] = mapped_column(
+        UUID(as_uuid=True), nullable=True,
+        comment="Currently viewed branch. No FK — soft pointer to avoid circular FK with session_branches",
+    )
+    handoff_count: Mapped[int] = mapped_column(
+        Integer, nullable=False, default=0,
+        comment="Number of times this session has been handed off",
+    )
+    total_active_seconds: Mapped[int] = mapped_column(
+        Integer, nullable=False, default=0,
+        comment="Cumulative active time in seconds",
+    )
+    total_parked_seconds: Mapped[int] = mapped_column(
+        Integer, nullable=False, default=0,
+        comment="Cumulative parked time in seconds",
+    )
+
     # ── Relationships ──
     user: Mapped["User"] = relationship("User", foreign_keys=[user_id])
     account: Mapped["Account"] = relationship("Account")
@@ -218,3 +244,18 @@ class AISession(Base):
         cascade="all, delete-orphan",
         order_by="AISessionStep.step_order",
     )
+    branches: Mapped[list["SessionBranch"]] = relationship(
+        "SessionBranch",
+        foreign_keys="SessionBranch.session_id",
+        cascade="all, delete-orphan",
+        order_by="SessionBranch.branch_order",
+    )
+    handoffs: Mapped[list["SessionHandoff"]] = relationship(
+        "SessionHandoff",
+        cascade="all, delete-orphan",
+        order_by="SessionHandoff.created_at",
+    )
+    resolution_outputs: Mapped[list["SessionResolutionOutput"]] = relationship(
+        "SessionResolutionOutput",
+        cascade="all, delete-orphan",
+    )