Add Fly release image and local start command

do

@@ -0,0 +1,59 @@
+#!/usr/bin/env python3
+"""Run the local app and its frontend watcher with one command."""
+
+import os
+import signal
+import socket
+import subprocess
+import sys
+import time
+from pathlib import Path
+
+
+ROOT = Path(__file__).resolve().parent
+
+
+def start():
+    with socket.socket() as probe:
+        if probe.connect_ex(("127.0.0.1", 8778)) == 0:
+            raise SystemExit("port 8778 is already in use")
+
+    commands = [
+        (["mise", "exec", "--", "python", "manage.py", "runserver", "8778"], ROOT),
+        (["mise", "exec", "--", "npx", "shadow-cljs", "watch", "app"], ROOT / "frontend"),
+    ]
+    children = []
+    stopping = False
+
+    def stop(_signal, _frame):
+        nonlocal stopping
+        stopping = True
+
+    signal.signal(signal.SIGINT, stop)
+    signal.signal(signal.SIGTERM, stop)
+    try:
+        for command, directory in commands:
+            children.append(subprocess.Popen(command, cwd=directory, start_new_session=True))
+        print("arthur: http://localhost:8778  (Ctrl-C stops both processes)", flush=True)
+        while not stopping:
+            for child in children:
+                if child.poll() is not None:
+                    raise SystemExit(f"app process exited with status {child.returncode}")
+            time.sleep(0.25)
+    finally:
+        for child in children:
+            if child.poll() is None:
+                os.killpg(child.pid, signal.SIGTERM)
+        for child in children:
+            try:
+                child.wait(timeout=5)
+            except subprocess.TimeoutExpired:
+                os.killpg(child.pid, signal.SIGKILL)
+                child.wait()
+
+
+if __name__ == "__main__":
+    if sys.argv[1:] == ["start"]:
+        start()
+    else:
+        raise SystemExit("usage: ./do start")