P2-ops: migration alignment, deploy test list, monitor alerts/single-host, backup/rollback closure, Caddy template

- migrations: MIGRATION_TABLES now equals POSTGRES_SCHEMA tables
  (uploads, task_attachments, task_followups, environments added);
  SEQUENCE_TABLES matches IDENTITY tables; task_followups
  sender_task_id column ensured before insert; UNMIGRATED_TABLES
  and migration_coverage() report tables with no PG schema instead
  of silently skipping them; CLI prints the coverage report.
- deploy: DEFAULT_TEST_SUITES gains deploy/tests (discover -s only,
  never package path); regression tests pin the default list.
- monitor: MONITOR_SINGLE_HOST=1 makes the server check single-host;
  default stays dual to avoid silent downgrade; server_mode is in
  the JSON. Optional fail-only alert outlet (webhook or mail,
  default off, alert body is pass/fail JSON only, no secrets).
- backup/rollback: README and restore-drill document what is
  recoverable (DB, code) vs not (worktrees, traces, uploads, host
  configs); rollback.sh shares the server-unit list and dual-port
  health success standard with host_deploy.sh (sync test guards it).
- public entry: Caddyfile uses native {$DOMAIN}/{$ACME_EMAIL}
  variables plus a caddy.service template; README documents the
  steps and the current BT-nginx production status.

deploy/host_deploy.sh

@@ -133,6 +133,8 @@ worker_unit() {
 # server-2（8766）是 HA 阶段 2 加的；unit 存在才纳入停/起与 health
 # 检查，老主机没有它时退回单机行为。2026-10-27 首推教训：脚本只动
 # server-1 时，LB 上游的 server-2 仍跑旧内存代码，新旧混跑。
+# 与 rollback.sh 共享的 unit 清单约定（P2-ops）：改动清单时两脚本
+# 同步改，deploy/tests/test_p2_ops.py 会双向核对。
 SERVER_UNITS="codex-cloud-server"
 HEALTH_PORTS="8765"
 if systemctl cat codex-cloud-server-2 >/dev/null 2>&1; then