feat: agent variants — migration, API, service integration, frontend, tests

- Migration 027: agent_variants table with single-active enforcement,
  variant_id column on agent_performance_log
- API: full CRUD, clone from agent/variant, activate/deactivate,
  per-variant performance metrics and history endpoints
- Services: extractor, event classifier, thesis rewriter all wired
  to AgentConfigResolver with variant override support
- Frontend: variant list, comparison view, create/edit/clone forms,
  activate/delete actions on Agents page
- Tests: API tests + 5 property-based tests (single-active invariant,
  clone preservation, config resolution, slug determinism, update idempotence)
- Spec files for agent-variants feature

services/recommendation/worker.py

@@ -796,6 +796,7 @@ async def generate_recommendation(
             deterministic_thesis=deterministic_thesis,
             summary=summary,
             config=ollama_config,
+            pool=pool,
         )
         # If the LLM returned the same text as the deterministic thesis,
         # treat it as a no-op (fallback was used).