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

CLAUDE.md

@@ -240,7 +240,7 @@ A checkpoint includes:
 | 1 | `phase-1/core-framework` | FastAPI + LangGraph + React chat loop + PostgresSaver | COMPLETED (2026-03-30) |
 | 2 | `phase-2/multi-agent-safety` | Supervisor routing + interrupts + templates | COMPLETED (2026-03-30) |
 | 3 | `phase-3/openapi-discovery` | OpenAPI parsing + MCP generation + SSRF protection | COMPLETED (2026-03-30) |
-| 4 | `phase-4/analytics-replay` | Replay API + analytics dashboard | NOT STARTED |
+| 4 | `phase-4/analytics-replay` | Replay API + analytics dashboard | COMPLETED (2026-03-31) |
 | 5 | `phase-5/polish-demo` | Error hardening + demo prep + Docker deploy | NOT STARTED |
 
 Status values: `NOT STARTED` -> `IN PROGRESS` -> `COMPLETED (YYYY-MM-DD)`