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/myai-data/Data/MigrationConstants.cs

@@ -0,0 +1,11 @@
+namespace MyAi.Data;
+
+/// <summary>
+/// Schema constants used by MyAiDbContext and migrations.
+/// Centralized to avoid hardcoded strings and ensure consistency.
+/// </summary>
+public static class MigrationConstants
+{
+    public const string SchemaName = "myAi";
+    public const string MigrationTableName = "_Migrations";
+}