Externalize secrets: move jwt.secret and DB creds to env; add .env.example; docs

[sugan0tech]

• Description: Secrets are committed in application.properties and application-build.properties. Move to environment variables, add .env.example, and update README.
• Current state: jwt.secret and MySQL creds are hardcoded; JwtUtil expects Base64 decoding but the secret in properties looks hex, not Base64.
• Tasks:
  • Replace property usage with env: JWT_SECRET_BASE64, SPRING_DATASOURCE_*.
  • Provide .env.example with commented guidance; ensure .gitignore covers .env.
  • Update JwtUtil/docs to clarify the secret must be Base64.
  • Document production secret management (Docker secrets/K8s, etc.).
• Acceptance criteria:
  • App starts with only .env provided; removing secrets from properties does not break dev.
  • README documents required env vars and formats.
• References: src/main/resources/application*.properties, src/main/java/com/hari/gatherspace/config/JwtUtil.java.