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/Features/FeatureUsageQueryServiceTests.cs

@@ -1,6 +1,7 @@
 using MicCheck.Api.Data;
 using MicCheck.Api.Features;
-using Microsoft.EntityFrameworkCore;
+using MicCheck.Api.Tests.Unit.TestSupport;
+using Moq;
 using NUnit.Framework;
 
 namespace MicCheck.Api.Tests.Unit.Features;
@@ -8,26 +9,23 @@ namespace MicCheck.Api.Tests.Unit.Features;
 [TestFixture]
 public class FeatureUsageQueryServiceTests
 {
-    private MicCheckDbContext _db = null!;
+    private Mock<IMicCheckDbContext> _db = null!;
+    private List<FeatureUsageDaily> _usage = null!;
     private FeatureUsageQueryService _service = null!;
     private const int EnvironmentId = 1;
 
     [SetUp]
     public void SetUp()
     {
-        var options = new DbContextOptionsBuilder<MicCheckDbContext>()
-            .UseInMemoryDatabase(Guid.NewGuid().ToString())
-            .Options;
-        _db = new MicCheckDbContext(options);
-        _service = new FeatureUsageQueryService(_db);
+        _db = new Mock<IMicCheckDbContext>();
+        _usage = [];
+        _db.SetupDbSet(c => c.FeatureUsageDaily, _usage);
+        _service = new FeatureUsageQueryService(_db.Object);
     }
 
-    [TearDown]
-    public void TearDown() => _db.Dispose();
-
     private void SeedUsage(int environmentId, int featureId, string featureName, DateOnly date, long count)
     {
-        _db.FeatureUsageDaily.Add(new FeatureUsageDaily
+        _usage.Add(new FeatureUsageDaily
         {
             EnvironmentId = environmentId,
             FeatureId = featureId,
@@ -36,7 +34,6 @@ public class FeatureUsageQueryServiceTests
             Count = count,
             UpdatedAt = DateTimeOffset.UtcNow
         });
-        _db.SaveChanges();
     }
 
     [Test]