FORCE_HTTPS now documents all four effects (redirect, HSTS, CSP
upgrade-insecure-requests, secure cookie flag) and is clearly marked
optional. COOKIE_SECURE default updated to "auto" with explanation of
auto-derivation logic. TRUST_PROXY clarifies it's off in dev unless
set and is required for FORCE_HTTPS. charts/README.md gains FORCE_HTTPS
and TRUST_PROXY entries. README prose expanded to explain all three
vars and their interaction.
jubnl