Fix index creation

[dartcafe]

• add guards to avoid db errors on attempts to recreate existing unique indices
• same for FK constraints

fix #4619

[bmurphy-apescic]

This did not work, still got the same errors as before.

I used the following, which allowed for the app to be updated.

DROP INDEX uniq_shares;
DROP INDEX uniq_token;
DROP INDEX uniq_subscription;
DROP INDEX uniq_unprocessed;
DROP INDEX uniq_watch;
DROP INDEX uniq_preferences;