refactor(makefile): rename start → backend, add restart logic for api and dashboard

- `make backend` replaces `make start`; polls postgres with nc (up to 10s)
  instead of fixed sleep, kills any running uvicorn before starting fresh
- `make dashboard` kills any running observable preview before restarting
- Update all references in CLAUDE.md, README.md, SCOPE.md, state-hub/README.md,
  and dashboard/src/docs/live-data.md

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -88,7 +88,7 @@ Health check: `make check`
 ### Shortcut
 
 ```bash
-make start    # db + migrate + api (all in one)
+make backend  # db + migrate + api (restarts if already running)
 ```
 
 ### Dashboard