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/Users/UserService.cs

@@ -4,28 +4,28 @@ using Microsoft.EntityFrameworkCore;
 
 namespace MicCheck.Api.Users;
 
-public class UserService(MicCheckDbContext db, IPasswordHasher<User> passwordHasher)
+public class UserService(IMicCheckDbContext db, IPasswordHasher<User> passwordHasher)
 {
     public async Task<User?> FindByIdAsync(int userId, CancellationToken ct = default) =>
         await db.Users.FirstOrDefaultAsync(u => u.Id == userId && u.IsActive, ct);
 
-    public async Task<(User? User, bool EmailConflict)> UpdateProfileAsync(
+    public async Task<ProfileUpdateResult> UpdateProfileAsync(
         int userId, string firstName, string lastName, string email, CancellationToken ct = default)
     {
         var user = await db.Users.FirstOrDefaultAsync(u => u.Id == userId && u.IsActive, ct);
-        if (user is null) return (null, false);
+        if (user is null) return new ProfileUpdateResult(null, EmailConflict: false);
 
         if (!string.Equals(user.Email, email, StringComparison.OrdinalIgnoreCase))
         {
             var emailTaken = await db.Users.AnyAsync(u => u.Email == email && u.Id != userId, ct);
-            if (emailTaken) return (null, true);
+            if (emailTaken) return new ProfileUpdateResult(null, EmailConflict: true);
         }
 
         user.FirstName = firstName;
         user.LastName = lastName;
         user.Email = email;
         await db.SaveChangesAsync(ct);
-        return (user, false);
+        return new ProfileUpdateResult(user, EmailConflict: false);
     }
 
     public async Task<bool> ChangePasswordAsync(
@@ -42,3 +42,5 @@ public class UserService(MicCheckDbContext db, IPasswordHasher<User> passwordHas
         return true;
     }
 }
+
+public record ProfileUpdateResult(User? User, bool EmailConflict);

src/api/MicCheck.Api/Users/UsersController.cs

@@ -39,11 +39,11 @@ public class UsersController(UserService userService) : ControllerBase
             string.IsNullOrWhiteSpace(request.Email))
             return BadRequest("First name, last name, and email are required.");
 
-        var (user, emailConflict) = await userService.UpdateProfileAsync(
+        var result = await userService.UpdateProfileAsync(
             userId.Value, request.FirstName.Trim(), request.LastName.Trim(), request.Email.Trim(), ct);
 
-        if (emailConflict) return Conflict("Email is already in use.");
-        return user is null ? NotFound() : Ok(UserResponse.From(user));
+        if (result.EmailConflict) return Conflict("Email is already in use.");
+        return result.User is null ? NotFound() : Ok(UserResponse.From(result.User));
     }
 
     [HttpPost("me/change-password")]