feat: administration — users, roles, stats, audit

DB:
- login_history table (user_id, ip, user_agent, timestamp)
- Migration v2.5: alter users + create login_history

Auth:
- Premier utilisateur = admin (is_admin=1)
- _log_login() après chaque connexion (register, local-login, OAuth)

Backend (app/routers/admin.py):
- admin_required middleware (vérifie is_admin)
- GET  /api/admin/users     → liste users + stats par user
- POST /api/admin/users     → créer user
- PUT  /api/admin/users/:id → modifier (name, email, password, admin, active)
- DELETE /api/admin/users/:id → supprimer + cascade
- GET  /api/admin/stats     → totaux globaux
- GET  /api/admin/audit     → historique login

Frontend (settings.html):
- Nav Admin visible seulement si userIsAdmin
- Users & Roles: stats cards, create/edit/delete users, table complète
- Audit Log: historique login avec date, IP, user-agent
- CSS professionnel: table-wrap, admin-table, stat-card, role-badge, status-dot

app/main.py

@@ -12,7 +12,7 @@ from starlette.middleware.sessions import SessionMiddleware
 from app.config import settings
 from app.db import init_db
 from app.middleware.csrf import CSRFMiddleware
-from app.routers import dashboard, board, notes, api, auth, webhooks, collections, my_tasks, workspace, public_api
+from app.routers import dashboard, board, notes, api, auth, webhooks, collections, my_tasks, workspace, public_api, admin
 from app.services.gitea_client import gitea
 from app.services.webhook_outbound import init_webhook_tables
 
@@ -58,6 +58,7 @@ app.include_router(webhooks.router)
 app.include_router(collections.router)
 app.include_router(my_tasks.router)
 app.include_router(workspace.router)
+app.include_router(admin.router)
 app.include_router(public_api.router)
 
 app.mount("/static", StaticFiles(directory="static"), name="static")