feat: times in the reader's own zone; the audit log shows exact moments (milestone 490)

- users.time_zone (0034). An account without one takes the browser's zone
  on its first page (POST /account/time-zone/adopt, never over a choice made
  by hand); the session follows the account on every request.
- /account: who you are and the zone every page writes its times in,
  reached from your name in the sidebar.
- formatting.when: 'Today 06:58', 'Yesterday 23:41', 'Tue 6 Oct 14:12',
  '6 Oct 2025 14:12', 24-hour, in the reader's zone; |ago's hover is the
  exact local moment instead of UTC; tz_label() for column headers.
- Audit log: When (EDT) with exact times, one detail fact a line, on classes.
- tzdata in the image, and the airgapped smoke checks a zone resolves.

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

Dockerfile

@@ -16,6 +16,9 @@ RUN DEBIAN_FRONTEND=noninteractive apt-get update \
         # tini — PID 1: reaps the zombies ssh and ansible leave behind and
         # forwards SIGTERM to Steward so a stop is a graceful shutdown
         tini \
+        # tzdata — the zone database, so times show in each reader's own zone
+        # (milestone 490); Debian's slim base no longer carries it
+        tzdata \
     && rm -rf /var/lib/apt/lists/*
 
 # Upgrade pip to suppress the version notice