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

Models/Vector/PictureVector.cs

@@ -0,0 +1,12 @@
+namespace Foxel.Models.Vector;
+
+using Microsoft.Extensions.VectorData;
+
+public class PictureVector
+{
+    [VectorStoreKey] public int Id { get; set; }
+    [VectorStoreData] public string? Name { get; set; }
+
+    [VectorStoreVector(Dimensions: 1024, DistanceFunction = DistanceFunction.CosineSimilarity)]
+    public ReadOnlyMemory<float>? Embedding { get; set; }
+}