feat: add backup CLI commands, AI correction/modifier services, and timezone-aware date display

- Add `devplace backups` CLI subcommands (list, run, prune, clear) with job enqueueing and orphan cleanup
- Introduce `BACKUPS_DIR` and `BACKUP_STAGING_DIR` config paths for backup storage
- Implement `schedule_correction` and `schedule_modification` calls in content creation, comment creation, and comment editing flows
- Add `DEFAULT_CORRECTION_PROMPT` and `DEFAULT_MODIFIER_PROMPT` config constants for AI content processing
- Document timezone-aware date display using `local_dt`/`dt_ago` Jinja globals with client-side `Intl` localization
- Update README with AI content correction/modifier support in direct messages via `@ai` inline instructions
- Add `track_action(user["uid"], "vote")` call on upvote in `apply_vote`

devplacepy/templates/gist_detail.html

@@ -23,7 +23,7 @@
                 {% if is_admin(user) and author and author.get('role') %}
                 <span style="font-size: 0.75rem; color: var(--text-muted);">&middot; {{ author['role'] }}</span>
                 {% endif %}
-                <span style="font-size: 0.75rem; color: var(--text-muted); margin-left: 0.5rem;">&middot; {{ time_ago }}</span>
+                <span style="font-size: 0.75rem; color: var(--text-muted); margin-left: 0.5rem;">&middot; {{ dt_ago(gist.created_at) if gist.get('created_at') else time_ago }}</span>
             </div>
         </div>