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>

CLAUDE.md

@@ -63,8 +63,8 @@ make api                      # uvicorn on 127.0.0.1:8000 (docs at /docs)
 make dashboard                # Observable preview on :3000
 make check                    # curl /state/health
 
-# Shortcut: db + migrate + api
-make start
+# Shortcut: db + migrate + api (restarts if already running)
+make backend
 ```
 
 The MCP server is registered in `.mcp.json` at the repo root. After `make install` creates `.venv`, restart Claude Code for `/mcp` to show `state-hub`.