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/Features/FeatureEvaluationService.cs

@@ -6,7 +6,7 @@ using Microsoft.EntityFrameworkCore;
 namespace MicCheck.Api.Features;
 
 public class FeatureEvaluationService(
-    MicCheckDbContext db,
+    IMicCheckDbContext db,
     SegmentEvaluator segmentEvaluator,
     FlagCache flagCache,
     FeatureUsageMetrics usageMetrics)

src/api/MicCheck.Api/Features/FeatureSegmentService.cs

@@ -4,7 +4,7 @@ using Microsoft.EntityFrameworkCore;
 
 namespace MicCheck.Api.Features;
 
-public class FeatureSegmentService(MicCheckDbContext db)
+public class FeatureSegmentService(IMicCheckDbContext db)
 {
     public async Task<IReadOnlyList<FeatureSegmentResponse>> ListByFeatureAsync(
         int featureId, int environmentId, CancellationToken ct = default)

src/api/MicCheck.Api/Features/FeatureService.cs

@@ -6,7 +6,7 @@ using Microsoft.EntityFrameworkCore;
 
 namespace MicCheck.Api.Features;
 
-public class FeatureService(MicCheckDbContext db, AuditService auditService, WebhookQueue webhookQueue)
+public class FeatureService(IMicCheckDbContext db, AuditService auditService, WebhookQueue webhookQueue)
 {
     private const int MaxFeaturesPerProject = 400;
 

src/api/MicCheck.Api/Features/FeatureStateService.cs

@@ -5,7 +5,7 @@ using Microsoft.EntityFrameworkCore;
 
 namespace MicCheck.Api.Features;
 
-public class FeatureStateService(MicCheckDbContext db, WebhookQueue webhookQueue, AuditService auditService)
+public class FeatureStateService(IMicCheckDbContext db, WebhookQueue webhookQueue, AuditService auditService)
 {
     public async Task<IReadOnlyList<FeatureState>> ListByEnvironmentAsync(int environmentId, CancellationToken ct = default)
     {

src/api/MicCheck.Api/Features/FeatureUsageQueryService.cs

@@ -3,7 +3,7 @@ using Microsoft.EntityFrameworkCore;
 
 namespace MicCheck.Api.Features;
 
-public class FeatureUsageQueryService(MicCheckDbContext db)
+public class FeatureUsageQueryService(IMicCheckDbContext db)
 {
     public async Task<DashboardUsageResponse> GetDashboardUsageAsync(int environmentId, int days, CancellationToken ct = default)
     {

src/api/MicCheck.Api/Features/TagService.cs

@@ -3,7 +3,7 @@ using Microsoft.EntityFrameworkCore;
 
 namespace MicCheck.Api.Features;
 
-public class TagService(MicCheckDbContext db)
+public class TagService(IMicCheckDbContext db)
 {
     public async Task<IReadOnlyList<Tag>> ListByProjectAsync(int projectId, CancellationToken ct = default)
     {