Add request validation and tests for API input models

[dangerworm]

Summary

• add FluentValidation integration with custom invalid model responses for API endpoints
• add input validators and parameter checks across user, theme, and item history routes
• introduce xUnit test project covering validation edge cases for input models

Testing

• Not run (dotnet SDK not available in environment)

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Fix missing FluentValidation/MVC usings to compile

Program.cs now calls AddValidatorsFromAssemblyContaining, AddFluentValidationAutoValidation, and configures ApiBehaviorOptions, but the file has no using FluentValidation;, using FluentValidation.AspNetCore;, or using Microsoft.AspNetCore.Mvc;. These extension methods and types are not covered by the Web SDK implicit usings, so the project will not compile (IServiceCollection won’t have those methods and ApiBehaviorOptions/BadRequestObjectResult are unresolved). Add the required using directives or fully qualify the types to restore a successful build.

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