Microservice to bring 2FA to self hosted PDSes

Document PDS env vars + fetch PDS_JWT_SECRET early #53

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Open opened by and assigned to andresitorresm.com
targeting main from andresitorresm.com/pds-gatekeeper:aitorres/document-pds-env-vars
Commits 2

refactor: read PDS_JWT_SECRET centrally and fail early if not found
doc: add note about required PDS env vars to README