/care refreshes itself when dosing changes: a mode change, a resume or a stop sends an event on /care/events #618

From pull #604 (closed), the part of #586 that main does not have yet. smith's branch smith/586-close-the-loop-sensors-ph-ec-temperature has a working start (Doser.onChange, the care routes subscribing to it, and a test in care-dosing.test.ts). Reuse it.

Done when:

• [ ] Doser (src/doser.ts) gets an onChange(fn) hook, called after a mode change, a resume, a halt or a dose.
• [ ] The care routes (src/care-routes.ts) send one event on /care/events for each change, so an open /care page re-renders the Dosing card without a reload.
• [ ] Tests: each of the four changes sends exactly one event, and a guest's event stream carries nothing about dosing.

Files: src/doser.ts, src/care-routes.ts, src/care-dosing.test.ts. Not here: anything that makes a pump run on its own (the next issue).