feat: add SessionHandoff model for conversational branching

Introduces the session_handoffs table as a unified park/escalate event
log with intent, snapshot, AI assessment, artifacts, and PSA push
tracking — replacing ad-hoc escalation_package writes.

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

backend/app/models/session_handoff.py

@@ -0,0 +1,50 @@
+"""Session handoff model — unified park/escalate with history."""
+import uuid
+from datetime import datetime, timezone
+from typing import Optional, Any, TYPE_CHECKING
+
+from sqlalchemy import String, Text, Boolean, DateTime, ForeignKey, CheckConstraint
+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.user import User
+
+
+class SessionHandoff(Base):
+    """A handoff event — either parking or escalating a session.
+    Dual-writes to ai_sessions.escalation_package for backward compat.
+    """
+    __tablename__ = "session_handoffs"
+    __table_args__ = (
+        CheckConstraint("intent IN ('park', 'escalate')", name="ck_session_handoffs_intent"),
+        CheckConstraint("priority IN ('normal', 'elevated')", name="ck_session_handoffs_priority"),
+    )
+
+    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)
+    handed_off_by: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), ForeignKey("users.id", ondelete="CASCADE"), nullable=False)
+    intent: Mapped[str] = mapped_column(String(20), nullable=False)
+    source_branch_id: Mapped[Optional[uuid.UUID]] = mapped_column(UUID(as_uuid=True), ForeignKey("session_branches.id", ondelete="SET NULL"), nullable=True)
+    snapshot: Mapped[dict[str, Any]] = mapped_column(JSONB, nullable=False, default=dict, comment="Branch map, status, next step, waiting on, watch out")
+    ai_assessment: Mapped[Optional[str]] = mapped_column(Text, nullable=True)
+    ai_assessment_data: Mapped[Optional[dict[str, Any]]] = mapped_column(JSONB, nullable=True, comment="{likely_cause, suggested_steps, confidence}")
+    artifacts: Mapped[Optional[list[dict[str, Any]]]] = mapped_column(JSONB, nullable=True, comment="[{name, type, reference}]")
+    engineer_notes: Mapped[Optional[str]] = mapped_column(Text, nullable=True)
+    priority: Mapped[str] = mapped_column(String(20), nullable=False, default="normal")
+    claimed_by: Mapped[Optional[uuid.UUID]] = mapped_column(UUID(as_uuid=True), ForeignKey("users.id", ondelete="SET NULL"), nullable=True)
+    claimed_at: Mapped[Optional[datetime]] = mapped_column(DateTime(timezone=True), nullable=True)
+    psa_note_pushed: Mapped[bool] = mapped_column(Boolean, default=False)
+    psa_note_id: Mapped[Optional[str]] = mapped_column(String(100), nullable=True)
+    notification_sent: Mapped[bool] = mapped_column(Boolean, default=False)
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=lambda: datetime.now(timezone.utc))
+
+    # Relationships
+    session: Mapped["AISession"] = relationship("AISession")
+    handed_off_by_user: Mapped["User"] = relationship("User", foreign_keys=[handed_off_by])
+    source_branch: Mapped[Optional["SessionBranch"]] = relationship("SessionBranch")
+    claimed_by_user: Mapped[Optional["User"]] = relationship("User", foreign_keys=[claimed_by])