COOKIE_SECURE now defaults to None and auto-derives from ENVIRONMENT
(production → true, else false) via a Pydantic model_validator. Explicit
env var values are still respected as an override escape hatch. Adds a
startup log line showing the resolved value. Restructures .env.example
with clear sections and inline docs, removes redundant production
checklist block.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>