Make the listen port configurable via PORT

app.py's dev server and gunicorn both hardcoded 5000, which is awkward
locally — macOS ControlCenter already holds that port, so the app can't
bind on the dev laptop without editing source.

Both now read PORT, defaulting to 5000 so EXPOSE, the compose
`${HOST_PORT}:5000` mapping and the deployed container are unchanged.
Read at run time rather than import time so a dev harness can assign a
free port.

Also tracks .claude/launch.json with autoPort set. It doesn't actually
launch on this machine — the preview sandbox can't read files under
~/Desktop — so CLAUDE.md notes that running from a shell with an
explicit PORT is the working local path.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.claude/launch.json

@@ -0,0 +1,12 @@
+{
+  "version": "0.0.1",
+  "configurations": [
+    {
+      "name": "arr-summary",
+      "runtimeExecutable": "python3",
+      "runtimeArgs": ["app.py"],
+      "port": 5000,
+      "autoPort": true
+    }
+  ]
+}