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

src/api/MicCheck.Api/Data/MicCheckDbContext.cs

@@ -13,7 +13,7 @@ using AppEnvironment = MicCheck.Api.Environments.Environment;
 
 namespace MicCheck.Api.Data;
 
-public class MicCheckDbContext : DbContext
+public class MicCheckDbContext : DbContext, IMicCheckDbContext
 {
     public MicCheckDbContext(DbContextOptions<MicCheckDbContext> options) : base(options) { }
 
@@ -42,3 +42,31 @@ public class MicCheckDbContext : DbContext
     protected override void OnModelCreating(ModelBuilder modelBuilder)
         => modelBuilder.ApplyConfigurationsFromAssembly(typeof(MicCheckDbContext).Assembly);
 }
+
+public interface IMicCheckDbContext
+{
+    DbSet<Organization> Organizations { get; }
+    DbSet<OrganizationUser> OrganizationUsers { get; }
+    DbSet<Project> Projects { get; }
+    DbSet<AppEnvironment> Environments { get; }
+    DbSet<Feature> Features { get; }
+    DbSet<FeatureState> FeatureStates { get; }
+    DbSet<FeatureSegment> FeatureSegments { get; }
+    DbSet<Tag> Tags { get; }
+    DbSet<Segment> Segments { get; }
+    DbSet<SegmentRule> SegmentRules { get; }
+    DbSet<SegmentCondition> SegmentConditions { get; }
+    DbSet<Identity> Identities { get; }
+    DbSet<IdentityTrait> IdentityTraits { get; }
+    DbSet<AuditLog> AuditLogs { get; }
+    DbSet<Webhook> Webhooks { get; }
+    DbSet<WebhookDeliveryLog> WebhookDeliveryLogs { get; }
+    DbSet<ApiKey> ApiKeys { get; }
+    DbSet<User> Users { get; }
+    DbSet<RefreshToken> RefreshTokens { get; }
+    DbSet<UserProjectPermission> UserProjectPermissions { get; }
+    DbSet<FeatureUsageDaily> FeatureUsageDaily { get; }
+
+    int SaveChanges();
+    Task<int> SaveChangesAsync(CancellationToken cancellationToken = default);
+}