Update user docs for Central v2026.1

[matthew-white]

• Upgrade instructions
• Merge Web Forms docs PRs before release
• Changes related to DB_* vs. PG* variables: DB config, including port, through libpq env vars central#1647
  • There are more variables available now compared to before, including PGPORT.
  • users with customization may need to revise those (e.g. port included in undocumented way)

[matthew-white]

@nicoleorlowski, feel free to edit the list above and add more items. I was thinking getodk/central#1337 and getodk/central#1338 should probably be included as well?

[brontolosone]

In re:

Changes related to DB_* vs. PG* variables: DB config, including port, through libpq env vars central#1647

we may want to consider:

• mentioning that for certain SSL configuration, for instance for certificate verification, you would need to make a certificate available inside the container, and while this can be done by modifying the Docker files, we don't provide anything specific for this yet, but talk to us on the support forum if you have such a need and need help.

[brontolosone]

when getodk/central-backend#1760 is in, we may want to add a section on making database backups. Perhaps under central-best-practices. Perhaps tutorial-style.

I'm thinking we should document, not necessarily in this order:

• Explain that there's a legacy .zip format and since 2026.1 a new, streamable, more flexible/standard format, and that restoring from the legacy format will still be possible for quite a while.
• Show how to make a backup (obviously)
• New: Show how to verify a backup (basically, pipe through openssl decryption, pipe into pg_restore, discard output but read its exit status). Nice, previously there was no way to see if your backup was corrupt, now there is! Unverifiable backups are not a good thing!
• With the above it's implicit that one doesn't need Central to get something usable out from the backup again, but we can make it explicit. Sysadmins will love this.
• Explain that with that standard pg_dump format, one has all the options from the pg_restore universe to do more advanced restores (I dunno, skip certain extensions, or just get one specific table out, or...) than what our lib/bin/restore.js allows. Sysadmins will love this.
• Show how to stream into lib/bin/restore.js in the dockerized setup. Explain that the legacy .zip format is not streamable. Explain that for that format one needs to get the file in somewhere accessible. Demo how that can be done ad-hoc.

I'm volunteering for a first pass for this, and then perhaps @lognaturel can edit out my garrulous documentation style? 😆