DOCSP-55438 & DOCSP-55529: Unify .env vars and add new verification scripts for all three backends

[cbullinger]

1. Created self-contained requirements verification scripts for each backend
2. Standardized environment variables across all three backends
3. Updated README files with "Verify Requirements" instructions

ℹ️ NOTE: The scripts will NOT fully succeed when run in the monorepo unless you change the SERVER_DIR in each script's configuration section. They are written and intended to be run from the root of each respective artifact repo, so you'll need to update them to:

• server/js-express
• server/java-spring
• server/python-fastapi