Improve database connection configuration and documentation for SSL

Update `docker-compose.yml` and `server/db.ts` to allow dynamic configuration of PostgreSQL SSL connections via the `DATABASE_SSL` environment variable. Add `DOCKER-DATABASE-SSL-GUIDE.md` detailing SSL options and troubleshooting.

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docker-compose.yml

@@ -34,6 +34,9 @@ services:
       NODE_ENV: production
       DATABASE_URL: postgresql://${POSTGRES_USER:-taskflow}:${POSTGRES_PASSWORD:-taskflow_password}@postgres:5432/${POSTGRES_DB:-taskflow}
       PORT: 5000
+      # DATABASE_SSL options: 'true' (SSL with self-signed), 'require' (SSL with valid cert), or unset (no SSL)
+      # For local postgres container, leave unset. For external PostgreSQL, set as needed.
+      DATABASE_SSL: ${DATABASE_SSL:-}
     ports:
       - "${APP_PORT:-5000}:5000"
     depends_on: