Support "every N days/weeks/months/years" recurrence

[codeling]

Summary

• Recurrence used to be one column holding a number of days, which can't express "every 3 weeks" or "every 10 years" (30 days is not a month, 365 days is not a year).
• todo.recurrenceMode is now the unit (0 none, 1 day, 2 week, 3 month, 4 year) and the new todo.recurrenceInterval holds N (1–999).
• Both columns stay on the todo table. Recurrence is 1:1 with a todo and read on every list query, so a separate table would only add a join.
• The edit dialog has a number field next to a unit select, in English and German.
• The next due date is computed in SQL with DATE_ADD(..., INTERVAL n UNIT). The helper recurrenceNextSql() is in the new queries/recurrence.php.
• update.php validates the unit and the interval.

Migration

sql/update-012.sql adds the column and converts the old day values. For example 7 becomes 1 week, 14 becomes 2 weeks, 30 becomes 1 month, 91 becomes 3 months and 365 becomes 1 year. Values that match nothing stay as days. sql/install.sql has the new schema and db.version is now 12.

Testing

• Updated the existing API tests to the new fields.
• Added a test covering each unit and the validation errors.
• PHP lint and node --check pass.
• Not run: the DB-backed tests and the migration, because there was no MySQL in my environment. Please run npm test and try update-012.sql on an old-schema copy.

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