Refactor reactions to track per-user instead of aggregate count

Replace Reaction.Count with UserId foreign key to User entity,
add unique index on (MessageId, EmojiName, UserId) to prevent
duplicates, and configure the User-Reactions relationship.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/DiscordArchiveManager/Data/DiscordArchiveContext.cs

@@ -111,7 +111,13 @@ public class DiscordArchiveContext : DbContext
                 .WithMany(m => m.Reactions)
                 .HasForeignKey(e => e.MessageId)
                 .OnDelete(DeleteBehavior.Cascade);
+            entity.HasOne(e => e.User)
+                .WithMany(u => u.Reactions)
+                .HasForeignKey(e => e.UserId)
+                .OnDelete(DeleteBehavior.Restrict);
             entity.HasIndex(e => e.MessageId);
+            entity.HasIndex(e => e.UserId);
+            entity.HasIndex(e => new { e.MessageId, e.EmojiName, e.UserId }).IsUnique();
         });
 
         // Mention configuration

src/DiscordArchiveManager/Models/Entities/Reaction.cs

@@ -19,9 +19,14 @@ public class Reaction
     [MaxLength(256)]
     public string EmojiName { get; set; } = null!;
 
-    public int Count { get; set; }
+    [Required]
+    [MaxLength(32)]
+    public string UserId { get; set; } = null!;
 
     // Navigation properties
     [ForeignKey(nameof(MessageId))]
     public Message Message { get; set; } = null!;
+
+    [ForeignKey(nameof(UserId))]
+    public User User { get; set; } = null!;
 }

src/DiscordArchiveManager/Models/Entities/User.cs

@@ -22,4 +22,5 @@ public class User
     public ICollection<UserSnapshot> Snapshots { get; set; } = new List<UserSnapshot>();
     public ICollection<Message> Messages { get; set; } = new List<Message>();
     public ICollection<Mention> Mentions { get; set; } = new List<Mention>();
+    public ICollection<Reaction> Reactions { get; set; } = new List<Reaction>();
 }