feat(backup): SET GLOBAL read_only quiesce + PostgreSQL & native DB support

[andrewbakercloudscale]

Summary

Three related improvements to how pi2s3 quiesces a database for a consistent, zero-downtime backup, plus agent-runnable install docs.

1. MySQL/MariaDB: SET GLOBAL read_only instead of FTWRL

Switched the quiesce from FLUSH TABLES WITH READ LOCK to SET GLOBAL read_only=ON (+ super_read_only=ON on MySQL, best-effort — MariaDB has no such variable). Gentler than holding a global read lock and needs no keepalive connection. Held only for the sub-10s sync window, then restored before imaging.

Safety: reads the prior read_only state and only flips it when OFF, so a replica that is already read-only is left untouched. A sentinel file recovers a stale read-only state left by a hard-killed run; the on-exit trap restores read-write on any normal or error exit.

2. PostgreSQL support (zero downtime)

New db_lock_postgres() issues a CHECKPOINT then images the live filesystem — writes are never blocked. WAL crash-recovery makes the single-volume snapshot (data dir incl. pg_wal) consistent on restore. New DB_PG_USER config.

3. Native (non-Docker) DB detection

DB_CONTAINER="auto" now detects native mariadbd/mysqld/postgres host processes, not just Docker containers. Native MySQL/MariaDB previously fell back to stop-the-service downtime; it now uses the zero-downtime path (set DB_ROOT_PASSWORD). New db_resolve_target() resolves engine + location and dispatches.

4. Agent-runnable install

install.sh gains a non-interactive env-var mode (S3_BUCKET/S3_REGION/NTFY_URL + PI2S3_YES). Added AGENTS.md and pi2s3.com/llms.txt so an AI agent pointed at the repo/site can "backup my site with pi2s3" unattended.

Docs (README, config.env.example, website) updated to reflect the real <10s write window and the new Postgres/native support.

Testing

• shellcheck -x clean on pi-image-backup.sh, lib/containers.sh, website/install (install.sh: only 2 pre-existing SC1090); bash -n passes on all (the CI gate).
• Unit-tested with mocks: find_db/container_db_engine (8 cases), db_resolve_target (11 branches), db_lock_mysql/db_unlock read-only logic (normal, replica-safe, stale-sentinel recovery, verify-fail, query-fail), and confirm() (4 modes incl. EOF-safe under set -e).
• Not exercised against a live DB / real S3 (no Linux+partclone+AWS env available locally).

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