auth, docs: a session from a longer window is cut; the remaining nits (audit A6, A7, A10, W7)

Security audit 2026-10-17, stage 6.

A10: a session created under a longer PANEL_SESSION_IDLE_DAYS kept its old
expiry after the setting was lowered (the renewal arithmetic took
ExpiresAt minus the new window for the last renewal). Touch now cuts such
a session to now plus the window at once. TestSessionMadeUnderALongerWindowIsCut.

A6, A7, W7 - security.md now says what the code does, which is what it
should do: a wrong method answers 405 with Allow from the router before the
role is looked at (the routes are public source); the setup token is made
anew at the first /setup/ request after expiry, a stranger's included, and
logged where only the operator reads it; two GETs make what must exist,
the server's own DKIM key on Settings and Health. README's local trial
says the compose file is the hardened deployment.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -125,6 +125,11 @@ docker run --rm -d --name selfpost-try \
   ghcr.io/mixeme/selfpost:2.14.3
 ```
 
+This trial runs with Docker's default capabilities and no
+`no-new-privileges`; the compose file of a real deployment
+(`deploy/docker-compose.yml`) drops every capability but the few it names,
+sets `no-new-privileges` and a process ceiling.
+
 **Get the setup URL** (pick one):
 
 ```sh