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/TestSupport/TestAsyncEnumerable.cs

@@ -0,0 +1,29 @@
+using System.Collections;
+using System.Linq.Expressions;
+
+namespace MicCheck.Api.Tests.Unit.TestSupport;
+
+public class TestAsyncEnumerable<T> : EnumerableQuery<T>, IAsyncEnumerable<T>, IQueryable<T>
+{
+    public TestAsyncEnumerable(IEnumerable<T> enumerable) : base(enumerable) { }
+
+    public TestAsyncEnumerable(Expression expression) : base(expression) { }
+
+    public IAsyncEnumerator<T> GetAsyncEnumerator(CancellationToken cancellationToken = default) =>
+        new TestAsyncEnumerator<T>(this.AsEnumerable().GetEnumerator());
+
+    IQueryProvider IQueryable.Provider => new TestAsyncQueryProvider<T>(this);
+}
+
+public sealed class TestAsyncEnumerator<T>(IEnumerator<T> inner) : IAsyncEnumerator<T>
+{
+    public T Current => inner.Current;
+
+    public ValueTask<bool> MoveNextAsync() => ValueTask.FromResult(inner.MoveNext());
+
+    public ValueTask DisposeAsync()
+    {
+        inner.Dispose();
+        return ValueTask.CompletedTask;
+    }
+}