add database backup and restore admin page

- SQLCipher-encrypted backup creation via VACUUM INTO
- Backup history with auto-pruning (keeps last 5)
- Pre-restore automatic backup for safety
- Restore from history or uploaded file
- Stats display with table breakdown
- Download hook for client-side file download
- SECRET_KEY_DB config for encryption at rest

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/berrypod_web/components/layouts/admin.html.heex

@@ -177,6 +177,14 @@
                 <.icon name="hero-arrow-uturn-right" class="size-5" /> Redirects
               </.link>
             </li>
+            <li>
+              <.link
+                navigate={~p"/admin/backup"}
+                class={admin_nav_active?(@current_path, "/admin/backup")}
+              >
+                <.icon name="hero-circle-stack" class="size-5" /> Backup
+              </.link>
+            </li>
           </ul>
         </div>
       </nav>