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/app/main.py

@@ -10,6 +10,8 @@ from typing import TYPE_CHECKING
 from fastapi import FastAPI, WebSocket, WebSocketDisconnect
 from fastapi.staticfiles import StaticFiles
 
+from app.analytics.api import router as analytics_router
+from app.analytics.event_recorder import NoOpAnalyticsRecorder
 from app.callbacks import TokenUsageCallbackHandler
 from app.config import Settings
 from app.db import create_checkpointer, create_pool, setup_app_tables
@@ -18,9 +20,10 @@ from app.graph import build_graph
 from app.intent import LLMIntentClassifier
 from app.interrupt_manager import InterruptManager
 from app.llm import create_llm
-from app.registry import AgentRegistry
-from app.session_manager import SessionManager
 from app.openapi.review_api import router as openapi_router
+from app.registry import AgentRegistry
+from app.replay.api import router as replay_router
+from app.session_manager import SessionManager
 from app.ws_handler import dispatch_message
 
 if TYPE_CHECKING:
@@ -73,6 +76,7 @@ async def lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
     app.state.escalator = escalator
     app.state.settings = settings
     app.state.pool = pool
+    app.state.analytics_recorder = NoOpAnalyticsRecorder()
 
     logger.info(
         "Smart Support started: %d agents loaded, LLM=%s/%s, template=%s",
@@ -87,9 +91,11 @@ async def lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
     await pool.close()
 
 
-app = FastAPI(title="Smart Support", version="0.3.0", lifespan=lifespan)
+app = FastAPI(title="Smart Support", version="0.4.0", lifespan=lifespan)
 
 app.include_router(openapi_router)
+app.include_router(replay_router)
+app.include_router(analytics_router)
 
 
 @app.websocket("/ws")