fix(migration): allow multiple agents per user (one per agent_type)

- Drop single-column UNIQUE constraint on user_agents.user_id
- Add composite UNIQUE constraint on (user_id, agent_type)
- Enables trigger to insert 3 agents per user (INTENT_RECOGNITION, TASK_EXECUTION, RESULT_REPORTING)
- Maintain reversible downgrade path

backend/alembic/versions/50ae013ce530_add_user_agent_catalog.py

@@ -20,6 +20,12 @@ depends_on: Union[str, Sequence[str], None] = None
 
 
 def upgrade() -> None:
+    op.drop_constraint("uq_user_agents_user_id", "user_agents", type_="unique")
+
+    op.create_unique_constraint(
+        "uq_user_agents_user_id_agent_type", "user_agents", ["user_id", "agent_type"]
+    )
+
     op.create_table(
         "user_agent_catalog",
         sa.Column("agent_type", sa.String(20), nullable=False),
@@ -132,6 +138,12 @@ def downgrade() -> None:
         $$;
     """)
 
+    op.drop_constraint(
+        "uq_user_agents_user_id_agent_type", "user_agents", type_="unique"
+    )
+
+    op.create_unique_constraint("uq_user_agents_user_id", "user_agents", ["user_id"])
+
     _drop_rls("user_agent_catalog")
     op.drop_constraint(
         "chk_user_agent_catalog_status", "user_agent_catalog", type_="check"