Merge pull request #58 from RandomProgramm3r/develop

refactor(core, business): unify pagination and promo serializers with enhanced validation

Introduces a shared pagination serializer and consolidates promo-related serializers,
validation logic, and country-code handling to improve structure, DRY-ness, and maintainability.

Key changes:
- **Core pagination**  
  - Add `BaseLimitOffsetPaginationSerializer` to centralize `limit`/`offset` validation for all endpoints.

- **Promo serializers overhaul**  
  - Collapse `PromoValidator` into serializers to co‑locate validation and data representation.  
  - Create `BasePromoSerializer` to extract common fields and methods, reducing duplication across create, detail, and read-only serializers.  
  - Implement `CountryField` and `MultiCountryField` for reusable ISO 3166‑1 α‑2 code validation.  
  - Simplify `PromoListQuerySerializer` by inheriting from the base pagination serializer and using `MultiCountryField`.

- **Enhanced promo validation**  
  - Support partial updates (`PATCH`), inferring `mode` from instance when absent in request data.  
  - Prevent lowering `max_count` below the current `used_count`.  
  - Ensure `get_used_codes_count` returns accurate counts for `COMMON`‑mode promotions.

Author: RandomProgramm3r

promo_code/business/models.py

@@ -119,8 +119,7 @@ def get_comment_count(self) -> int:
     def get_used_codes_count(self) -> int:
         if self.mode == business.constants.PROMO_MODE_UNIQUE:
             return self.unique_codes.filter(is_used=True).count()
-        # TODO: COMMON Promo
-        return 0
+        return self.used_count
 
     @property
     def get_available_unique_codes(self) -> list[str] | None: