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/MockDbSetFactory.cs

@@ -0,0 +1,56 @@
+using System.Linq.Expressions;
+using MicCheck.Api.Data;
+using Microsoft.EntityFrameworkCore;
+using Moq;
+
+namespace MicCheck.Api.Tests.Unit.TestSupport;
+
+public static class MockDbSetFactory
+{
+    public static Mock<DbSet<T>> Create<T>(List<T> data) where T : class
+    {
+        var queryable = data.AsQueryable();
+        var mockSet = new Mock<DbSet<T>>();
+
+        mockSet.As<IAsyncEnumerable<T>>()
+            .Setup(m => m.GetAsyncEnumerator(It.IsAny<CancellationToken>()))
+            .Returns(() => new TestAsyncEnumerator<T>(data.GetEnumerator()));
+
+        mockSet.As<IQueryable<T>>().Setup(m => m.Provider).Returns(() => new TestAsyncQueryProvider<T>(data.AsQueryable().Provider));
+        mockSet.As<IQueryable<T>>().Setup(m => m.Expression).Returns(() => data.AsQueryable().Expression);
+        mockSet.As<IQueryable<T>>().Setup(m => m.ElementType).Returns(queryable.ElementType);
+        mockSet.As<IQueryable<T>>().Setup(m => m.GetEnumerator()).Returns(() => data.GetEnumerator());
+
+        mockSet.Setup(m => m.Add(It.IsAny<T>())).Callback<T>(item => data.Add(item));
+        mockSet.Setup(m => m.AddRange(It.IsAny<IEnumerable<T>>())).Callback<IEnumerable<T>>(items => data.AddRange(items));
+        mockSet.Setup(m => m.Remove(It.IsAny<T>())).Callback<T>(item => data.Remove(item));
+        mockSet.Setup(m => m.RemoveRange(It.IsAny<IEnumerable<T>>())).Callback<IEnumerable<T>>(items =>
+        {
+            foreach (var item in items.ToList())
+                data.Remove(item);
+        });
+
+        return mockSet;
+    }
+
+    public static void SetupDbSet<T>(this Mock<IMicCheckDbContext> context, Expression<Func<IMicCheckDbContext, DbSet<T>>> selector, List<T> data)
+        where T : class =>
+        context.Setup(selector).Returns(Create(data).Object);
+
+    /// <summary>
+    /// Sets up a DbSet whose Add() simulates database-generated identity assignment (needed because
+    /// Moq does not run SaveChanges and entity Id properties are commonly init-only).
+    /// </summary>
+    public static void SetupDbSetWithGeneratedIds<T>(this Mock<IMicCheckDbContext> context, Expression<Func<IMicCheckDbContext, DbSet<T>>> selector, List<T> data)
+        where T : class
+    {
+        var idProperty = typeof(T).GetProperty("Id") ?? throw new InvalidOperationException($"{typeof(T).Name} has no Id property.");
+        var mockSet = Create(data);
+        mockSet.Setup(m => m.Add(It.IsAny<T>())).Callback<T>(item =>
+        {
+            idProperty.SetValue(item, data.Count + 1);
+            data.Add(item);
+        });
+        context.Setup(selector).Returns(mockSet.Object);
+    }
+}