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_main.py

@@ -13,7 +13,7 @@ class TestMainModule:
         assert app.title == "Smart Support"
 
     def test_app_version(self) -> None:
-        assert app.version == "0.3.0"
+        assert app.version == "0.4.0"
 
     def test_agents_yaml_path_exists(self) -> None:
         assert AGENTS_YAML.name == "agents.yaml"
@@ -25,3 +25,11 @@ class TestMainModule:
     def test_websocket_route_registered(self) -> None:
         routes = [r.path for r in app.routes if hasattr(r, "path")]
         assert "/ws" in routes
+
+    def test_replay_router_registered(self) -> None:
+        routes = [r.path for r in app.routes if hasattr(r, "path")]
+        assert any("replay" in p or "conversations" in p for p in routes)
+
+    def test_analytics_router_registered(self) -> None:
+        routes = [r.path for r in app.routes if hasattr(r, "path")]
+        assert any("analytics" in p for p in routes)