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/Common/Security/ApiKeys/ApiKeyEndpoints.cs

@@ -13,10 +13,9 @@ public static class ApiKeyEndpoints
 
         group.MapPost("/api-keys", async (int organizationId, CreateApiKeyRequest request, ApiKeyService apiKeyService, CancellationToken ct) =>
         {
-            var (key, rawKey) = await apiKeyService.CreateAsync(
-                organizationId, request.Name, request.ExpiresAt, ct);
+            var result = await apiKeyService.CreateAsync(organizationId, request.Name, request.ExpiresAt, ct);
 
-            return Results.Ok(new CreateApiKeyResponse(key.Id, key.Name, rawKey, key.Prefix, key.ExpiresAt));
+            return Results.Ok(new CreateApiKeyResponse(result.Key.Id, result.Key.Name, result.RawKey, result.Key.Prefix, result.Key.ExpiresAt));
 
         }).WithName("CreateApiKey");