✨ Activity-based browsing

DysonNetwork.Sphere/Activity/ActivityService.cs

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+using DysonNetwork.Sphere.Post;
+using NodaTime;
+
+namespace DysonNetwork.Sphere.Activity;
+
+public class ActivityService(AppDatabase db)
+{
+    public async Task<Activity> CreateActivity(
+        Account.Account user,
+        string type,
+        string identifier,
+        ActivityVisibility visibility = ActivityVisibility.Public
+    )
+    {
+        var activity = new Activity
+        {
+            Type = type,
+            ResourceIdentifier = identifier,
+            Visibility = visibility,
+            AccountId = user.Id,
+        };
+
+        db.Activities.Add(activity);
+        await db.SaveChangesAsync();
+
+        return activity;
+    }
+
+    public async Task CreateNewPostActivity(Account.Account user, Post.Post post)
+    {
+        if (post.Visibility is PostVisibility.Unlisted or PostVisibility.Private) return;
+
+        var identifier = $"posts/{post.Id}";
+        await CreateActivity(user, "posts.new", identifier,
+            post.Visibility == PostVisibility.Friends ? ActivityVisibility.Friends : ActivityVisibility.Public);
+    }
+}
+
+public static class ActivityQueryExtensions
+{
+    public static IQueryable<Activity> FilterWithVisibility(this IQueryable<Activity> source,
+        Account.Account? currentUser, List<long> userFriends)
+    {
+        var now = Instant.FromDateTimeUtc(DateTime.UtcNow);
+
+        if (currentUser is null)
+            return source.Where(e => e.Visibility == ActivityVisibility.Public);
+
+        return source
+            .Where(e => e.Visibility != ActivityVisibility.Friends ||
+                        userFriends.Contains(e.AccountId) ||
+                        e.AccountId == currentUser.Id);
+    }
+}