Move RAG repository from rag-api to rag-data — consolidate data layer ownership

- Move IRagRepository, EfRagRepository, and VectorSerializer from rag-api/Data to rag-data/Repositories
- Add rag-api-models ProjectReference to rag-data.csproj for model type availability
- Delete rag-api/Data folder (no longer needed; all data access is now in rag-data)
- This aligns RAG with email-api and other services: all data code in the data project

Pattern: rag-api (API logic) → rag-data (repository, EF entities, migrations)

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Apis/rag-data/Repositories/VectorSerializer.cs

@@ -0,0 +1,31 @@
+namespace Rag.Data.Repositories;
+
+public static class VectorSerializer
+{
+    public static byte[] ToBytes(float[] vector)
+    {
+        var bytes = new byte[vector.Length * sizeof(float)];
+        Buffer.BlockCopy(vector, 0, bytes, 0, bytes.Length);
+        return bytes;
+    }
+
+    public static float[] FromBytes(byte[] bytes)
+    {
+        var vector = new float[bytes.Length / sizeof(float)];
+        Buffer.BlockCopy(bytes, 0, vector, 0, bytes.Length);
+        return vector;
+    }
+
+    public static double CosineSimilarity(float[] a, float[] b)
+    {
+        if (a.Length == 0 || a.Length != b.Length) return 0;
+        double dot = 0, magA = 0, magB = 0;
+        for (var i = 0; i < a.Length; i++)
+        {
+            dot += a[i] * b[i];
+            magA += a[i] * a[i];
+            magB += b[i] * b[i];
+        }
+        return magA == 0 || magB == 0 ? 0 : dot / (Math.Sqrt(magA) * Math.Sqrt(magB));
+    }
+}