feat(admin): add admin backend endpoints and service layer

Why:
- Dashboard needs system health, Qdrant management, chunk preview, questions, and cost estimation endpoints

Changes:
- Add admin router with 11 endpoints (health, Qdrant CRUD, chunk preview, questions management, cost estimation)
- Add delete_experiment to SQLite storage
- Mount admin router and dashboard static files at /app

Impact:
- New /admin/* API routes available
- Dashboard served at /app/ via StaticFiles

src/main.py

@@ -1,6 +1,7 @@
 """FastAPI app factory. Composes all domain routers and middleware."""
 
 import logging
+import os
 import time
 from starlette.middleware.base import BaseHTTPMiddleware
 from starlette.requests import Request
@@ -8,6 +9,7 @@ from starlette.responses import Response
 
 from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
+from fastapi.staticfiles import StaticFiles
 
 from src.core.exceptions import (
     ChunkingError,
@@ -18,6 +20,7 @@ from src.core.exceptions import (
 )
 from src.documents.routes import router as documents_router
 from src.benchmarking.routes import router as benchmarking_router
+from src.admin.routes import router as admin_router
 from src.storage.sqlite import init_db
 
 # Configure logging
@@ -91,6 +94,11 @@ def create_app() -> FastAPI:
     # Mount domain routers
     app.include_router(documents_router)
     app.include_router(benchmarking_router)
+    app.include_router(admin_router)
+
+    # Mount dashboard at /app
+    static_dir = os.path.join(os.path.dirname(os.path.abspath(__file__)), "static")
+    app.mount("/app", StaticFiles(directory=static_dir, html=True), name="dashboard")
 
     return app