feat: add abuse protection with throttler + logging to Postgres

[phertyameen]

Description

Adds abuse protection to critical APIs using @nestjs/throttler and logs blocked attempts into Postgres for auditing.

Related Issues

Closes issue #351

Changes Made

• Integrated @nestjs/throttler to protect critical APIs
• Added AbuseLog entity + service to record abuse attempts into Postgres
• Implemented custom LoggingThrottlerGuard to auto-log blocked requests

How to Test

1. Run the app with npm run start:dev.
2. Hit the login endpoint 6+ times within a minute.
3. Confirm:
   • The 6th request returns a rate-limit error (HTTP 429).
   • A record is created in the abuse_logs table with endpoint, ip, reason.

Next Steps

• Apply @Throttle() decorators to future auth, purchase, and transfer modules once they are created.

Screenshots (if applicable)

Checklist

• My code follows the project's coding style.
• I have tested these changes locally.
• Documentation has been updated where necessary.

Additional note for future Contributors

• Login/Auth, Purchase, and Transfer modules are not yet created.
• Once these modules are added, apply @Throttle decorators (or rely on global guard) to enforce limits.
• Abuse logs can be viewed in the abuse_logs table for monitoring suspicious activity.