fix(security): Register a global ValidationPipe

[baranylcn]

No global ValidationPipe is registered (missing in main.ts, no APP_PIPE). Only 4 controllers validate input via @UsePipes (meetings, tasks, subtasks, tags). The other 10 - auth, users, api-keys, communities, clubs, activity-feed, notifications, notification-preferences, moderation, gamification - accept unvalidated bodies (no whitelist, DTO decorators ignored).

To do

• Register app.useGlobalPipes(new ValidationPipe({ whitelist: true, transform: true })) in main.ts.
• Remove now-redundant per-controller @UsePipes.