Feature/global jwt auth guard

[Glam26]

Here's a summary of the changes made:

1. Registered JwtAuthGuard Globally:
   • Created the @public() decorator at public.decorator.ts.
   • Updated jwt-auth.guard.ts to utilize Reflector so it skips endpoints
   marked with the @public() decorator.
   • Updated main.ts to register the JwtAuthGuard globally via
   app.useGlobalGuards(new JwtAuthGuard(app.get(Reflector))) .
2. Added a Controller-Level Test:
   • Created global-guard.e2e-spec.ts with two endpoints: one protected by
   default
   and one exposed by @public() . The test verifies that unauthenticated
   requests to the protected route return 401 Unauthorized , while the
   public route works perfectly.
3. Documented the Pattern:
   • Created auth.md explaining how the global JWT guard works and
   provides an example for explicit opt-outs using the @public()
   decorator.
   closes [backend][security] Auth module's JWT verification is not registered globally #530

[Xhristin3]

LGTM