Replace the hardcoded 1440-minute (24h) JWT lifetime with
DECNET_JWT_EXP_MINUTES (validated positive int, default 240 = 4h).
Shrinks the passive window of a stolen token; active revocation is
unchanged (immediate->=<10s).
Reference template for .env / .env.local showing every variable that
decnet/env.py consumes, with short rationale per section (system
logging, embedded workers, profiling, API server, …). Copy to .env
and fill in secrets; .env itself stays gitignored.