feat: Refactor database configuration to use granular environment variables; update Docker and CI/CD workflows accordingly

.env.example

@@ -1,4 +1,14 @@
 # Example environment variables for CalMiner
 
-# PostgreSQL database connection URL
-database_url=postgresql://<user>:<password>@localhost:5432/calminer
+# PostgreSQL connection settings
+DATABASE_DRIVER=postgresql
+DATABASE_HOST=localhost
+DATABASE_PORT=5432
+DATABASE_USER=<user>
+DATABASE_PASSWORD=<password>
+DATABASE_NAME=calminer
+# Optional: set a schema (comma-separated for multiple entries)
+# DATABASE_SCHEMA=public
+
+# Legacy fallback (still supported, but granular settings are preferred)
+# DATABASE_URL=postgresql://<user>:<password>@localhost:5432/calminer