fix: add back_populates to SessionHandoff and SessionResolutionOutput relationships

SQLAlchemy SAWarning about overlapping relationships was promoted to
an error by pytest filterwarnings=error, crashing mapper initialization
and causing 500s on every request — cascading to 423+ test failures.

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

backend/app/models/ai_session.py

@@ -253,10 +253,12 @@ class AISession(Base):
     )
     handoffs: Mapped[list["SessionHandoff"]] = relationship(
         "SessionHandoff",
+        back_populates="session",
         cascade="all, delete-orphan",
         order_by="SessionHandoff.created_at",
     )
     resolution_outputs: Mapped[list["SessionResolutionOutput"]] = relationship(
         "SessionResolutionOutput",
+        back_populates="session",
         cascade="all, delete-orphan",
     )