feat(vector-db): integrate vector database for image search

- Replace pgvector with Microsoft Vector DB for image vector storage and search
- Update Picture model to use float[] instead of Vector type
- Modify PictureService to use VectorDbService for vector search
- Remove vector-related code from MyDbContext
- Add PictureVector model for Vector DB integration

MyDbContext.cs

@@ -5,17 +5,6 @@ namespace Foxel;
 
 public class MyDbContext(DbContextOptions<MyDbContext> options) : DbContext(options)
 {
-    protected override void OnModelCreating(ModelBuilder modelBuilder)
-    {
-        modelBuilder.HasPostgresExtension("vector");
-
-        modelBuilder.Entity<Picture>()
-            .HasIndex(p => p.Embedding)
-            .HasMethod("ivfflat")
-            .HasOperators("vector_cosine_ops")
-            .HasStorageParameter("lists", 100);
-    }
-
     public DbSet<Picture> Pictures { get; set; } = null!;
     public DbSet<User> Users { get; set; } = null!;
     public DbSet<Tag> Tags { get; set; } = null!;