feat: complete phase 4 -- conversation replay API + analytics dashboard

- Replay models: StepType enum, ReplayStep, ReplayPage frozen dataclasses
- Checkpoint transformer: PostgresSaver JSONB -> structured timeline steps
- Replay API: GET /api/conversations (paginated), GET /api/replay/{thread_id}
- Analytics models: AgentUsage, InterruptStats, AnalyticsResult
- Analytics event recorder: Protocol + PostgresAnalyticsRecorder + NoOp
- Analytics queries: resolution_rate, agent_usage, escalation_rate, cost, interrupts
- Analytics API: GET /api/analytics?range=Xd with envelope response
- DB migration: analytics_events table + conversations column additions
- 74 new tests, 399 total passing, 92.87% coverage

backend/tests/unit/test_db.py

@@ -55,7 +55,7 @@ class TestDbModule:
         from app.db import setup_app_tables
 
         await setup_app_tables(mock_pool)
-        assert mock_conn.execute.await_count == 2
+        assert mock_conn.execute.await_count == 4
 
     def test_ddl_statements_valid(self) -> None:
         assert "CREATE TABLE IF NOT EXISTS conversations" in _CONVERSATIONS_DDL