Add REST API endpoint for role management

[dauglyon]

Summary

• Adds new endpoint POST /api/V2/admin/users/{username}/roles for programmatic role assignment
• Supports both built-in roles (Admin, DevToken, ServToken, CreateAdmin) and custom roles
• Uses add/remove semantics for atomic updates

Request Body

{
  "addRoles": ["Admin", "DevToken"],
  "removeRoles": ["ServToken"],
  "addCustomRoles": ["custom_role_id"],
  "removeCustomRoles": ["old_role"]
}

Test plan

• Run ./gradlew test and verify all tests pass
• Start local server and test endpoint manually with curl
• Verify 403 response when non-admin attempts role assignment
• Verify 400 response for invalid role names

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.44%. Comparing base (77a783a) to head (37c0606).

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #510      +/-   ##
=============================================
+ Coverage      93.40%   93.44%   +0.03%     
- Complexity      2158     2173      +15     
=============================================
  Files            129      129              
  Lines           7615     7658      +43     
  Branches        1170     1178       +8     
=============================================
+ Hits            7113     7156      +43     
  Misses           459      459              
  Partials          43       43              

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