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/Environments/EnvironmentDocumentService.cs

@@ -5,7 +5,7 @@ using AppEnvironment = MicCheck.Api.Environments.Environment;
 
 namespace MicCheck.Api.Environments;
 
-public class EnvironmentDocumentService(MicCheckDbContext db)
+public class EnvironmentDocumentService(IMicCheckDbContext db)
 {
     public async Task<EnvironmentDocumentResponse?> GetAsync(int environmentId, CancellationToken ct = default)
     {

src/api/MicCheck.Api/Environments/EnvironmentService.cs

@@ -7,7 +7,7 @@ using AppEnvironment = MicCheck.Api.Environments.Environment;
 
 namespace MicCheck.Api.Environments;
 
-public class EnvironmentService(MicCheckDbContext db, AuditService auditService)
+public class EnvironmentService(IMicCheckDbContext db, AuditService auditService)
 {
     public async Task<IReadOnlyList<AppEnvironment>> ListByProjectAsync(int projectId, CancellationToken ct = default)
     {

src/api/MicCheck.Api/Environments/EnvironmentsController.cs

@@ -141,8 +141,8 @@ public class EnvironmentsController(EnvironmentService environmentService, Webho
         var environment = await environmentService.FindByApiKeyAsync(apiKey, ct);
         if (environment is null) return NotFound();
 
-        var (_, logs) = await auditLogQueryService.ListByEnvironmentAsync(environment.Id, new Audit.AuditLogFilter(), ct);
-        return Ok(logs.ToList());
+        var result = await auditLogQueryService.ListByEnvironmentAsync(environment.Id, new Audit.AuditLogFilter(), ct);
+        return Ok(result.Items.ToList());
     }
 
     [HttpGet("api/v1/environment/{apiKey}/identities")]
@@ -157,9 +157,9 @@ public class EnvironmentsController(EnvironmentService environmentService, Webho
         var environment = await environmentService.FindByApiKeyAsync(apiKey, ct);
         if (environment is null) return NotFound();
 
-        var (total, items) = await adminIdentityService.ListAsync(environment.Id, page, pageSize, ct);
-        var results = items.Select(Identities.AdminIdentityResponse.From).ToList();
-        return Ok(new PaginatedResponse<Identities.AdminIdentityResponse>(total, null, null, results));
+        var result = await adminIdentityService.ListAsync(environment.Id, page, pageSize, ct);
+        var results = result.Items.Select(Identities.AdminIdentityResponse.From).ToList();
+        return Ok(new PaginatedResponse<Identities.AdminIdentityResponse>(result.Total, null, null, results));
     }
 
     [HttpPost("api/v1/environment/{apiKey}/identities")]