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/TestAsyncQueryProvider.cs

@@ -0,0 +1,43 @@
+using System.Linq.Expressions;
+using Microsoft.EntityFrameworkCore.Query;
+
+namespace MicCheck.Api.Tests.Unit.TestSupport;
+
+public class TestAsyncQueryProvider<T>(IQueryProvider innerProvider) : IAsyncQueryProvider
+{
+    public IQueryable CreateQuery(Expression expression) =>
+        new TestAsyncEnumerable<T>(StripIncludes(expression));
+
+    public IQueryable<TElement> CreateQuery<TElement>(Expression expression) =>
+        new TestAsyncEnumerable<TElement>(StripIncludes(expression));
+
+    public object? Execute(Expression expression) =>
+        innerProvider.Execute(StripIncludes(expression));
+
+    public TResult Execute<TResult>(Expression expression) =>
+        innerProvider.Execute<TResult>(StripIncludes(expression));
+
+    public TResult ExecuteAsync<TResult>(Expression expression, CancellationToken cancellationToken = default)
+    {
+        var stripped = StripIncludes(expression);
+        var resultType = typeof(TResult).GetGenericArguments() is [var inner] ? inner : typeof(TResult);
+        var executionResult = typeof(IQueryProvider)
+            .GetMethod(nameof(IQueryProvider.Execute), 1, [typeof(Expression)])!
+            .MakeGenericMethod(resultType)
+            .Invoke(innerProvider, [stripped]);
+
+        return (TResult)typeof(Task).GetMethod(nameof(Task.FromResult))!
+            .MakeGenericMethod(resultType)
+            .Invoke(null, [executionResult])!;
+    }
+
+    private static Expression StripIncludes(Expression expression) => new IncludeStripVisitor().Visit(expression);
+
+    private sealed class IncludeStripVisitor : ExpressionVisitor
+    {
+        protected override Expression VisitMethodCall(MethodCallExpression node) =>
+            node.Method.Name is "Include" or "ThenInclude"
+                ? Visit(node.Arguments[0])
+                : base.VisitMethodCall(node);
+    }
+}