[Backend] Add a global `ValidationPipe` in `main.ts`

[Muhammadcodes112]

Description

Without this, DTO validation decorators (class-validator) on request bodies/queries do nothing — this one line is what makes them actually enforce.

Where

astre_server/src/main.ts

What to implement

app.useGlobalPipes(new ValidationPipe({ whitelist: true, forbidNonWhitelisted: true, transform: true }));

added before app.listen(...).

Acceptance Criteria

• The pipe is registered globally in main.ts
• A request with an invalid field (per any DTO's class-validator decorators, e.g. GET /pools?status=NotARealStatus) now returns 400 instead of passing through unvalidated
• npm run build compiles cleanly

Hints

• This is a one-line change but unblocks the acceptance criteria of several other issues (like "Implement PoolsController GET /pools"'s 400-on-invalid-status requirement) — worth doing early.