Reopen handled notifications on same-reason new activity

[blink-claw-bot]

Problem

The recent re-open fix only reopens a dismissed notification when the GitHub notification reason changes. That catches cases like subscribed -> mention, but it can miss same-reason new activity, such as a new comment on a subscribed thread that remains reason=subscribed.

If GitHub returns an already-handled notification in the active notifications API again, that is itself useful signal: there is unread/new activity for the thread.

Suggested behavior

Reopen a locally handled notification when GitHub serves it again with new activity, even if reason did not change.

Implementation hints

Options:

• Store GitHub updated_at on the notification row and reopen when incoming updated_at > last_handled_github_updated_at.
• Store last_read_at/latest_comment_url where available and compare those.
• Simpler first pass: if an existing row is dismissed=1 or acted_at IS NOT NULL and appears in gh api notifications, clear dismissed, clear acted_at, and set action_taken = 'RE-OPENED: new GitHub activity detected'.

Be careful not to reset rows that are currently processing=1; those should probably stay locked.