Apply rate limiting to deep-link endpoints (#1327)

[lamborghini21]

Summary

Applies request throttling to sensitive endpoints in src/deep-link/deep-link.controller.ts to protect against brute-force and abuse, per #1327.

Changes

• Added ThrottlerGuard + @Throttle to the deep-link course redirect endpoint, with a stricter limit than the module default.
• Registered ThrottlerModule.forRoot in deep-link.module.ts.
• Added deep-link.config.ts to centralize rate-limit values — configurable via env vars (DEEP_LINK_RATE_LIMIT_TTL_MS, DEEP_LINK_RATE_LIMIT_DEFAULT, DEEP_LINK_RATE_LIMIT_REDIRECT) with sane defaults, no hardcoded magic numbers.
• Added deep-link.controller.spec.ts covering:
  • Normal requests succeed within the limit.
  • Exceeding the limit returns 429 with a clear message.

Acceptance Criteria

• Exceeding the limit returns HTTP 429.
• Limits are documented and configurable, not hardcoded magic numbers.
• Normal usage is unaffected.

Testing

Closes #1327

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