Adds `npm run maint:check` (health report) and `npm run maint:reindex`
(REINDEX DATABASE + REFRESH COLLATION VERSION + ANALYZE) for post-
upgrade maintenance, modelled after Nextcloud's occ maintenance.
Documented in README.
Also relaxes postgres image from digest pin back to tag-pin
(pgvector/pgvector:pg16) — the auto-REINDEX-on-drift check in
database.js and the COLLATE "C" protection on critical indexes
make the digest pin redundant while blocking ordinary `compose
pull` updates.
