Update the webauthn_credential_id_sequence in Postgres

[zeripath]

There is (yet) another problem with v210 in that Postgres will silently allow preset
ID insertions ... but it will not update the sequence value.

This PR simply adds a little step to the end of the v210 migration to update the
sequence number.

Users who have already migrated who find that they cannot insert new
webauthn_credentials into the DB can either run:

gitea doctor recreate-table webauthn_credential

or in postgres run:

SELECT setval('webauthn_credential_id_seq', COALESCE((SELECT MAX(id)+1 FROM `webauthn_credential`), 1), false)

which will fix the bad sequence.

Fix #19012

Signed-off-by: Andrew Thornton art27@cantab.net

[zeripath]

An alternative option is to add yet another migration v211 which will simply run the alter sequence command then we can drop the rest of the warning and make it automatic.

Actually let me just do that. On second thoughts maybe not - I'm gonna let reviewers decide that. The problem with adding this as a new migration is:

• Imagine a server which was not affected primarily because no insertions took place in v210.
• Users have since been able to insert and then delete webauthn_credentials. Imagine the last few credentials inserted have been deleted.
• The max ID in the webauthn_credential would be say n, whereas the sequence would actually be n+k.
• An additional migration would set the sequence back to n+1.

This is a low risk problem and should cause no issues but it is one worth considering.

[6543]

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