Fix millisecond/second mismatch in SQLite task visibility timeout

[djgrant]

Fixes#32

SqliteTaskQueue.process() wrote visible_from as a millisecond timestamp while the DAO's visibility queries compared it against strftime('%s', 'now') (seconds), so a task claimed by a crashed worker stayed hidden ~55,000 years instead of five minutes.

Changes:

• TaskQueueDao.getNextTask() and getTaskCount() now take a millisecond now value from JS and compare with visible_from <= $now, so all values share one unit and one clock.
• Inserts set visible_from = 0 explicitly (matching makeVisible), so the column default no longer matters — existing databases created with the old seconds-based default keep working.
• Exported SqliteTaskQueue from @yieldstar/sqlite-runtime and added the regression test suggested in the issue: claim a task, advance past the visibility window, and confirm a fresh queue can reclaim it.