feat: add startWecker and stopWecker mutations to start/stop alarm via API

- Add startWecker mutation: starts wecker.py via subprocess if not already ringing
- Add stopWecker mutation: kills the running wecker process via SIGTERM if ringing
- Both mutations handle the 'already ringing' / 'not ringing' edge cases gracefully
- Update README with API documentation for the new mutations
- Add comprehensive tests (unit + GraphQL endpoint)

README.md

@@ -99,6 +99,21 @@ mutation {
 }
 ```
 
+**Start the alarm immediately:**
+```graphql
+mutation {
+  startWecker
+}
+```
+Returns `true` if the alarm started, `false` if it was already ringing (ignored).
+
+**Stop the alarm immediately:**
+```graphql
+mutation {
+  stopWecker
+}
+```
+Returns `true` if the alarm was stopped, `false` if it wasn't ringing.
 
 ### Example response for `isRinging`:
 ```json