fix: add database name variables for backward compatibility

existing installations use 'postgres' database to preserve data
new installations use service-specific databases (postiz, waha, lightrag)

.env.example

@@ -323,6 +323,13 @@ POSTGRES_DB=postgres
 POSTGRES_PORT=5432
 POSTGRES_USER=postgres
 
+# Service-specific database names
+# New installations use dedicated databases per service
+# Existing installations may use 'postgres' for backward compatibility
+POSTIZ_DB_NAME=postiz
+WAHA_DB_NAME=waha
+LIGHTRAG_DB_NAME=lightrag
+
 ############
 # Supavisor -- Database pooler and others that can be left as default values
 ############