feat: add ForkPoint model for conversational branching

Introduces the fork_points table to capture decision points where a
FlowPilot session diverges into multiple diagnostic branches, storing
the trigger step, fork reason, and available options as JSONB.

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

backend/app/models/fork_point.py

@@ -0,0 +1,35 @@
+"""Fork point model — captures decision points where a session branches."""
+import uuid
+from datetime import datetime, timezone
+from typing import Optional, Any, TYPE_CHECKING
+
+from sqlalchemy import Text, DateTime, ForeignKey
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+from sqlalchemy.dialects.postgresql import UUID, JSONB
+
+from app.core.database import Base
+
+if TYPE_CHECKING:
+    from app.models.ai_session import AISession
+    from app.models.session_branch import SessionBranch
+    from app.models.ai_session_step import AISessionStep
+
+
+class ForkPoint(Base):
+    """A decision point where a session forks into multiple branches.
+    options JSONB: [{label, description, branch_id, status}]
+    """
+    __tablename__ = "fork_points"
+
+    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)
+    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)
+    options: Mapped[list[dict[str, Any]]] = mapped_column(JSONB, nullable=False, default=list, comment="[{label, description, branch_id, status}]")
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=lambda: datetime.now(timezone.utc))
+
+    # Relationships
+    session: Mapped["AISession"] = relationship("AISession")
+    parent_branch: Mapped["SessionBranch"] = relationship("SessionBranch")
+    trigger_step: Mapped[Optional["AISessionStep"]] = relationship("AISessionStep")