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>

steward/migrations/versions/0034_user_time_zone.py

@@ -0,0 +1,22 @@
+"""The zone each user reads times in (milestone 490)
+
+Revision ID: 0034_user_time_zone
+Revises: 0033_retire_uptime_widget
+Create Date: 2026-10-08
+"""
+from typing import Sequence, Union
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "0034_user_time_zone"
+down_revision: Union[str, None] = "0033_retire_uptime_widget"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column("users", sa.Column("time_zone", sa.String(64), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column("users", "time_zone")