Refactor nunit tests off InMemory/WebApplicationFactory, remove tuple returns (#4)

## Summary
- Extract `IMicCheckDbContext` and mock DB access with Moq instead of the EF InMemory provider, per CLAUDE.md testing guidelines.
- Rewrite HTTP-pipeline tests to exercise controllers/auth handlers directly instead of `WebApplicationFactory`.
- Remove tuple return types across the API in favor of named records (`PagedResult<T>`, `ProfileUpdateResult`, `ApiKeyCreationResult`).

## Test plan
- [x] `dotnet build` (full solution)
- [x] `dotnet test tests/api/MicCheck.Api.Tests.Unit` — 187/187 pass
- [x] `dotnet test tests/api/MicCheck.Api.Tests.Integration` — 8/8 pass (live API + Postgres via Aspire)
- [x] Verified Aspire AppHost/dashboard starts and API responds

Reviewed-on: #4

tests/api/MicCheck.Api.Tests.Unit/MicCheck.Api.Tests.Unit.csproj

@@ -11,8 +11,6 @@
 
   <ItemGroup>
     <PackageReference Include="coverlet.collector" Version="6.0.4" />
-    <PackageReference Include="Microsoft.AspNetCore.Mvc.Testing" Version="10.0.5" />
-    <PackageReference Include="Microsoft.EntityFrameworkCore.InMemory" Version="10.0.5" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.3.0" />
     <PackageReference Include="Moq" Version="4.20.72" />
     <PackageReference Include="NUnit" Version="4.5.1" />