feat: add SessionBranch model for conversational branching

Introduces the session_branches table to represent diagnostic hypothesis
paths within a FlowPilot session, supporting parent/child branch
relationships, status lifecycle, and per-branch conversation history.

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

backend/app/models/session_branch.py

@@ -0,0 +1,58 @@
+"""Session branch model — represents a diagnostic hypothesis path within a FlowPilot session."""
+import uuid
+from datetime import datetime, timezone
+from typing import Optional, Any, TYPE_CHECKING
+
+from sqlalchemy import String, Text, DateTime, ForeignKey, Integer, CheckConstraint, Index
+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.ai_session_step import AISessionStep
+    from app.models.user import User
+
+
+class SessionBranch(Base):
+    """A diagnostic branch within a FlowPilot session."""
+    __tablename__ = "session_branches"
+    __table_args__ = (
+        CheckConstraint(
+            "status IN ('active', 'dead_end', 'solved', 'untried', 'revived')",
+            name="ck_session_branches_status",
+        ),
+        CheckConstraint(
+            "branch_order > 0",
+            name="ck_session_branches_branch_order_positive",
+        ),
+        Index("ix_session_branches_session_id", "session_id"),
+        Index("ix_session_branches_parent_branch_id", "parent_branch_id"),
+        Index("ix_session_branches_session_status", "session_id", "status"),
+        Index("ix_session_branches_session_order", "session_id", "branch_order"),
+    )
+
+    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)
+    parent_branch_id: Mapped[Optional[uuid.UUID]] = mapped_column(UUID(as_uuid=True), ForeignKey("session_branches.id", ondelete="CASCADE"), nullable=True)
+    fork_point_step_id: Mapped[Optional[uuid.UUID]] = mapped_column(UUID(as_uuid=True), ForeignKey("ai_session_steps.id", ondelete="SET NULL"), nullable=True)
+    branch_order: Mapped[int] = mapped_column(Integer, nullable=False, default=1)
+    label: Mapped[str] = mapped_column(String(200), nullable=False)
+    status: Mapped[str] = mapped_column(String(20), nullable=False, default="active")
+    status_reason: Mapped[Optional[str]] = mapped_column(Text, nullable=True)
+    status_changed_at: Mapped[Optional[datetime]] = mapped_column(DateTime(timezone=True), nullable=True)
+    status_changed_by: Mapped[Optional[uuid.UUID]] = mapped_column(UUID(as_uuid=True), ForeignKey("users.id", ondelete="SET NULL"), nullable=True)
+    conversation_messages: Mapped[list[dict[str, Any]]] = mapped_column(JSONB, nullable=False, default=list, comment="LLM message history scoped to this branch")
+    context_summary: Mapped[Optional[dict[str, Any]]] = mapped_column(JSONB, nullable=True, comment="{tried: [], concluded: str, artifacts: []}")
+    evidence_from_branch_id: Mapped[Optional[uuid.UUID]] = mapped_column(UUID(as_uuid=True), ForeignKey("session_branches.id", ondelete="SET NULL"), nullable=True)
+    evidence_description: Mapped[Optional[str]] = mapped_column(Text, nullable=True)
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=lambda: datetime.now(timezone.utc))
+    updated_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=lambda: datetime.now(timezone.utc), onupdate=lambda: datetime.now(timezone.utc))
+
+    # Relationships
+    session: Mapped["AISession"] = relationship("AISession", foreign_keys=[session_id])
+    parent_branch: Mapped[Optional["SessionBranch"]] = relationship("SessionBranch", remote_side="SessionBranch.id", foreign_keys=[parent_branch_id])
+    fork_point_step: Mapped[Optional["AISessionStep"]] = relationship("AISessionStep", foreign_keys=[fork_point_step_id])
+    status_changed_by_user: Mapped[Optional["User"]] = relationship("User", foreign_keys=[status_changed_by])
+    evidence_source: Mapped[Optional["SessionBranch"]] = relationship("SessionBranch", remote_side="SessionBranch.id", foreign_keys=[evidence_from_branch_id])