feat(01-03): ProfileRepository and ProfileService with write-then-replace

- ProfileRepository: SemaphoreSlim write lock + write-then-replace (tmp→validate→move)
- ProfileRepository: camelCase JSON serialization matching existing schema
- ProfileService: CRUD operations (Add/Rename/Delete) with validation
- All 10 ProfileServiceTests pass (round-trip, missing file, corrupt JSON, concurrency, schema check)

SharepointToolbox/Services/ProfileService.cs

@@ -0,0 +1,54 @@
+using SharepointToolbox.Core.Models;
+using SharepointToolbox.Infrastructure.Persistence;
+
+namespace SharepointToolbox.Services;
+
+public class ProfileService
+{
+    private readonly ProfileRepository _repository;
+
+    public ProfileService(ProfileRepository repository)
+    {
+        _repository = repository;
+    }
+
+    public Task<IReadOnlyList<TenantProfile>> GetProfilesAsync()
+        => _repository.LoadAsync();
+
+    public async Task AddProfileAsync(TenantProfile profile)
+    {
+        if (string.IsNullOrWhiteSpace(profile.Name))
+            throw new ArgumentException("Profile name must not be empty.", nameof(profile));
+
+        if (string.IsNullOrWhiteSpace(profile.TenantUrl) ||
+            !Uri.TryCreate(profile.TenantUrl, UriKind.Absolute, out _))
+            throw new ArgumentException("TenantUrl must be a valid absolute URL.", nameof(profile));
+
+        if (string.IsNullOrWhiteSpace(profile.ClientId))
+            throw new ArgumentException("ClientId must not be empty.", nameof(profile));
+
+        var existing = (await _repository.LoadAsync()).ToList();
+        existing.Add(profile);
+        await _repository.SaveAsync(existing);
+    }
+
+    public async Task RenameProfileAsync(string existingName, string newName)
+    {
+        var profiles = (await _repository.LoadAsync()).ToList();
+        var target = profiles.FirstOrDefault(p => p.Name == existingName)
+            ?? throw new KeyNotFoundException($"Profile '{existingName}' not found.");
+
+        target.Name = newName;
+        await _repository.SaveAsync(profiles);
+    }
+
+    public async Task DeleteProfileAsync(string name)
+    {
+        var profiles = (await _repository.LoadAsync()).ToList();
+        var target = profiles.FirstOrDefault(p => p.Name == name)
+            ?? throw new KeyNotFoundException($"Profile '{name}' not found.");
+
+        profiles.Remove(target);
+        await _repository.SaveAsync(profiles);
+    }
+}