Add email invitation system with SMTP configuration and invite token workflow

Implemented comprehensive email invitation functionality for new users. Added SMTP settings management in admin panel with configuration UI for host, port, credentials, sender details, and TLS options. Created invite token system with 48-hour expiration and single-use enforcement. Added mailer module with HTML email templates in German theme styling (radial gradient backgrounds, gold accents, bordered cards). Implemented invite

backend/app/main.py

@@ -12,6 +12,7 @@ from .routes.auth import router as auth_router
 from .routes.admin import router as admin_router
 from .routes.games import router as games_router
 from .routes.setup import router as setup_router
+from .routes.invites import router as invites_router
 
 app = FastAPI(title="Cluedo Sheet")
 
@@ -31,6 +32,7 @@ app.include_router(auth_router)
 app.include_router(admin_router)
 app.include_router(games_router)
 app.include_router(setup_router)
+app.include_router(invites_router)
 
 
 def _rand_join_code(n: int = 6) -> str: