Add production dependencies introduced in recent security commits.
Add docs/ENVIRONMENT_VARIABLES.md documenting TRUST_PROXY, SECRET,
SESSION_SECRET, and ALLOWED_ORIGINS with configuration guidance.
Rewrites all 7 backend ADRs from a custom structure to the MADR 2.1.2
template required by the VS Code ADR Manager extension: bullet metadata
(Status/Date), standardised section headings, "Chosen option: X, because Y"
wording, and explicit Pros/Cons blocks per option.
julien