feat(psa): add psa_ticket_id and psa_connection_id to sessions

Add columns to link sessions to PSA tickets and connections. Includes
migration 059, model relationship, and response schema fields.

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

backend/alembic/versions/059_add_psa_ticket_link_to_sessions.py

@@ -0,0 +1,31 @@
+"""Add psa_ticket_id and psa_connection_id to sessions.
+
+Revision ID: 059
+Revises: 058
+"""
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+revision = "059"
+down_revision = "058"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column("sessions", sa.Column("psa_ticket_id", sa.String(100), nullable=True))
+    op.add_column(
+        "sessions",
+        sa.Column(
+            "psa_connection_id",
+            postgresql.UUID(as_uuid=True),
+            sa.ForeignKey("psa_connections.id", ondelete="SET NULL"),
+            nullable=True,
+        ),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("sessions", "psa_connection_id")
+    op.drop_column("sessions", "psa_ticket_id")