feat: add favicon.ico and gate model write APIs by env flag

- Generate and serve real /favicon.ico from static assets
- Update HTML to use /favicon.ico
- Add ENABLE_MODEL_RW_API setting (default: false)
- Disable POST/DELETE model endpoints by default
- Hide write endpoints from OpenAPI when disabled
- Return 404 for write endpoints when disabled
- Update env.example with ENABLE_MODEL_RW_API documentation
- Update README and PRD with R/W API policy and remote compose notes
- Add tests to verify write endpoints are disabled by default

env.example

@@ -26,6 +26,10 @@ API_PORT=8000
 # Numero di worker processes per uVicorn
 API_WORKERS=4
 
+# Abilita API R/W modelli (POST /pull, DELETE /models/{name})
+# Default sicuro: false (endpoint non disponibili)
+ENABLE_MODEL_RW_API=false
+
 # ===========================================
 # CORS Configuration
 # ===========================================