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/<squad_number> for partial updates.

• All fields are patchable except squad_number (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 squad_number or id, the endpoint returns 400 Bad Request.

Suggested Approach

1. Model — Add PlayerPatchRequest struct in src/models/player.rs with all patchable fields as Option<T>. Exclude squad_number and id.
2. Service — Add a patch function in src/services/player_service.rs that retrieves the existing player by squad_number and applies only the Some(…) fields.
3. Route — Add a #[patch("/players/squadnumber/<squad_number>")] handler in src/routes/players.rs. Check for forbidden fields → 400. Look up player → 404 if missing. Return Status::NoContent on success.
4. Mount — Register the new route in src/main.rs.
5. Tests — Add integration tests following the existing pattern.

Acceptance Criteria

• PATCH /players/squadnumber/<squad_number> is implemented
• All fields except squad_number 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 squad_number 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
• Prerequisite: [FEATURE] Replace integer IDs with UUIDs and promote squad number as natural key #38 (UUID surrogate key + squad number as natural key)