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>

dashboard/src/docs/live-data.md

@@ -41,7 +41,7 @@ To restart the API:
 cd ~/the-custodian/state-hub
 make api          # starts uvicorn on 127.0.0.1:8000
 # or, if postgres is not running:
-make start        # db + migrate + api
+make backend      # db + migrate + api (restarts if already running)
 ```
 
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