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

infra/migrations/027_agent_variants.sql

@@ -0,0 +1,47 @@
+-- Agent variant configurations: alternative model/prompt/parameter sets per agent.
+-- Each agent can have multiple variants for A/B testing, model comparison,
+-- and iterative prompt engineering. At most one variant per agent is active.
+
+CREATE TABLE IF NOT EXISTS agent_variants (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    agent_id UUID NOT NULL REFERENCES ai_agents(id) ON DELETE CASCADE,
+    variant_name VARCHAR(200) NOT NULL,
+    variant_slug VARCHAR(200) NOT NULL,
+    description TEXT NOT NULL DEFAULT '',
+    model_provider VARCHAR(50) NOT NULL DEFAULT 'ollama',
+    model_name VARCHAR(200) NOT NULL,
+    system_prompt TEXT NOT NULL DEFAULT '',
+    user_prompt_template TEXT NOT NULL DEFAULT '',
+    prompt_version VARCHAR(100) NOT NULL DEFAULT '',
+    temperature FLOAT DEFAULT 0.0,
+    max_tokens INTEGER DEFAULT 32768,
+    context_window INTEGER DEFAULT 0,       -- Ollama num_ctx; 0 = use model default
+    input_token_limit INTEGER DEFAULT 0,    -- max input tokens before truncation; 0 = no limit
+    token_budget INTEGER DEFAULT 0,         -- total tokens per hour; 0 = unlimited
+    timeout_seconds INTEGER DEFAULT 120,
+    max_retries INTEGER DEFAULT 2,
+    is_active BOOLEAN NOT NULL DEFAULT FALSE,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+-- Each agent can have many variants, but variant slugs must be unique per agent.
+CREATE UNIQUE INDEX IF NOT EXISTS idx_agent_variants_slug
+    ON agent_variants(agent_id, variant_slug);
+
+-- At most one active variant per agent (database-enforced invariant).
+CREATE UNIQUE INDEX IF NOT EXISTS idx_agent_variants_active
+    ON agent_variants(agent_id) WHERE is_active = TRUE;
+
+-- Fast lookup by agent.
+CREATE INDEX IF NOT EXISTS idx_agent_variants_agent
+    ON agent_variants(agent_id);
+
+-- Add variant_id to performance log for per-variant attribution.
+-- Nullable so existing rows are unaffected; ON DELETE SET NULL preserves
+-- historical log entries when a variant is removed.
+ALTER TABLE agent_performance_log
+    ADD COLUMN IF NOT EXISTS variant_id UUID REFERENCES agent_variants(id) ON DELETE SET NULL;
+
+CREATE INDEX IF NOT EXISTS idx_agent_perf_variant
+    ON agent_performance_log(variant_id, recorded_at DESC);