Implement `PATCH` method for partial updates

[nanotaboada]

Problem

The API supports PUT for player updates, which requires sending the complete player payload even when only one or two fields need to change. A PATCH endpoint would allow callers to update specific fields without providing the entire resource.

Proposed Solution

Add PATCH /players/{squadNumber} for partial updates (following the existing PUT and DELETE URL convention in this repo).

• All fields are patchable except squadNumber (natural key, present in the URL path) and id (UUID surrogate key).
• Only the fields present in the request body are updated; absent fields retain their current values.
• If the request body includes squadNumber or id, the endpoint returns 400 Bad Request.

Suggested Approach

1. DTO — Add PlayerPatchDTO in the models package with all patchable fields as nullable (e.g. String, Boolean). Exclude squadNumber and id. Annotate with @JsonInclude(JsonInclude.Include.NON_NULL) so absent fields serialize cleanly.
2. Service — Add boolean patch(Integer squadNumber, PlayerPatchDTO dto) in PlayersService. Retrieve the existing player, apply only the non-null fields from the DTO, then persist.
3. Controller — Add @PatchMapping("/players/{squadNumber}") in PlayersController. Check for forbidden fields (squadNumber, id) → 400. Look up player → 404 if missing. Return ResponseEntity<Void> with HttpStatus.NO_CONTENT on success.
4. Tests — Add unit tests following existing naming conventions.

Acceptance Criteria

• PATCH /players/{squadNumber} is implemented
• All fields except squadNumber and id are patchable
• Fields absent from the request body are left unchanged
• Returns 204 No Content on success
• Returns 400 Bad Request if the body contains squadNumber or id
• Returns 400 Bad Request on field validation failure
• Returns 404 Not Found when no player has that squad number
• Tests added following existing naming conventions
• All existing tests continue to pass
• CHANGELOG.md updated

References

• RFC 5789 — PATCH Method for HTTP
• RFC 7396 — JSON Merge Patch
• Spring — @PatchMapping