feat: Improve configuration loading robustness, add Vercel-specific fallbacks, and update documentation for config.json best practices.

.env.example

@@ -52,6 +52,9 @@ DS2API_ADMIN_KEY=admin
 
 # Option C: Base64 encoded JSON (recommended for Vercel env var)
 # DS2API_CONFIG_JSON=eyJrZXlzIjpbInlvdXItYXBpLWtleSJdLCJhY2NvdW50cyI6W3siZW1haWwiOiJ1c2VyQGV4YW1wbGUuY29tIiwicGFzc3dvcmQiOiJ4eHgiLCJ0b2tlbiI6IiJ9XX0=
+#
+# Generate from local config.json:
+#   DS2API_CONFIG_JSON="$(base64 < config.json | tr -d '\n')"
 
 # ---------------------------------------------------------------
 # Paths (optional)