[docs] Add database maintenance section; update info message on ANALYZE run (sqlite)

[tsmethurst]

Description

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The full ANAYLZE command we run for SQLite after each DB migration is not really necessary, and takes bloody ages (minutes and minutes on my 1cpu 1gb ram machine). This PR updates the post-migration ANALYZE command to use an analysis limit of 10000, which should be plenty (for comparison, we use 1000 for on-close callbacks).

Correction to the above: I've been messing about with running partial and full analyzes, and it turns out that running PRAGMA anaylsis_limit=10000; ANALYZE; actually doesn't cut it for the purposes of helping the query optimizer use the correct indexes.

Here's the contents of sqlite_stat1 for statuses after a full ANALYZE:

And here's the same after a partial ANALYZE:

Huge difference in emphasis across some of the important indexes.

Running the statuses web view query after a full analyze uses the intended index:

Where running the same query after a partial analyze causes the query planner to not use correct indexes:

So in other words, we should actually keep that full analyze after all!

Also adds some v. basic docs about db maintenance, for SQLite at least.

Checklist

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• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have not leveraged AI to create the proposed changes.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[NyaaaWhatsUpDoc]

if you're now going to not limit the size of the analyze run and instead using this for documentation changes, i think it'd be worth renaming the PR

[tsmethurst]

Will do yeah :)

[daenney]

Suggested change

	2. While connected to your GoToSocial database file in the `sqlite3` shell, run `PRAGMA analysis_limit=0; ANALYZE;` (this may take quite a few minutes).
	2. While connected to your GoToSocial database file in the `sqlite3` shell, run `ANALYZE;` (this may take quite a few minutes).

There's not much point to PRAGMA analysys_limit=0, it's the same thing as not setting it at all.