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

@@ -42,7 +42,7 @@ make api                      # uvicorn :8000 --reload
 ### Shortcut
 
 ```bash
-make start    # db + sleep + migrate + api
+make backend  # db + migrate + api (restarts if already running)
 ```
 
 ### Dashboard
@@ -74,7 +74,7 @@ custodian register-project    # register cwd as a Custodian project
 | `make api` | `uvicorn api.main:app --reload` |
 | `make dashboard` | Observable dev server |
 | `make check` | `curl /state/health` |
-| `make start` | `db` + wait + `migrate` + `api` |
+| `make backend` | `db` + wait + `migrate` + `api` (restarts if running) |
 | `make register-project DOMAIN=x PROJECT_PATH=y` | Register a project |
 | `make clean` | `docker compose down -v` (destroys DB volume) |