Security: Add HTTPS and TRUST_PROXY settings

The app only ever speaks plain HTTP, so it could not know the site was
served over TLS: generated links were http:// (the /webadmin bounce
from a blog host sent the login page over http), the session cookie
was never Secure and nothing sent HSTS. HTTPS=true fixes all three;
ClearSessionCookie now uses the same attributes as the set, since a
browser only replaces a cookie whose Secure flag matches.

TRUST_PROXY=true makes the client address the last X-Forwarded-For
entry — the one our proxy appended — so throttling and the log see
real addresses instead of the proxy's; earlier entries are whatever
the client sent and are ignored. Production startup warns when HTTPS
is off.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Sd8UPWrvyYCLj97JexNw3A

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