docs(readme): add missing Data→Repositories dependency in diagram

- Repositories consume DbContext (Data) at runtime via constructor
  injection; the edge was missing from the Layer 4 arrows

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

Author: nanotaboada

README.md

@@ -165,6 +165,7 @@ graph RL
     %% Layer 4
     Models --> Repositories
     Models --> Data
+    Data --> Repositories
     EFCore --> Data
     EFCore -.-> Repositories