docs: say what to back up, and why mydb.sqlite alone is not enough

[spelingbee]

One paragraph under Deployment saying what to back up: the whole data directory, because mydb.sqlite on its own is missing the rows still in its -wal and the files under uploads/ and output/ are not in the database at all. Tested on v0.18.0: restoring mydb.sqlite alone gives an instance that answers 403 at login; restoring the directory restores everything.

Closes #630.

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Summary by cubic

Adds backup guidance to the README explaining that the whole data directory must be backed up, not just mydb.sqlite.

• mydb.sqlite alone is missing the -wal file with recent rows and the uploads/ and output/ files, so restoring only it leaves an instance that returns 403 at login.
• Closes Docs: a line on what to back up - mydb.sqlite alone restores an instance you cannot log in to #630.

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