feat: /version endpoint; fix deploy action inputs

- Bake the commit SHA into the image (GIT_SHA build arg) and serve it at
  /version, so a deploy can be verified end to end. This repo is the
  dockerloid deploy test service.
- cssnr/portainer-stack-deploy-action@v1 reads `endpoint` and `pull`;
  `endpoint_id` / `pull_image` were silently ignored.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app.py

@@ -1,3 +1,5 @@
+import os
+
 from flask import Flask, jsonify, render_template, request
 
 import ascii_engine
@@ -15,6 +17,11 @@ def index():
     )
 
 
+@app.route("/version")
+def version():
+    return jsonify(gitSha=os.environ.get("GIT_SHA", "unknown"))
+
+
 @app.route("/api/random")
 def api_random():
     category = request.args.get("category")