feat: Bake build metadata (git tag, commit, date) into OCI image and display in web UI

Store git tag, commit hash, and build date as OCI-standard labels and a
build_meta.json file inside the Docker image. The Flask app reads this at
startup and displays version info in all page footers. Adds /version JSON
endpoint for programmatic access. Falls back to dev@local outside Docker.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app.py

@@ -72,6 +72,13 @@ def warmup_cache():
     logger.info("Cache warmup complete.")
 
 app = Flask(__name__)
+
+import json as _json
+_meta_path = Path(__file__).parent / "build_meta.json"
+BUILD_META = _json.loads(_meta_path.read_text()) if _meta_path.exists() else {
+    "tag": "dev", "commit": "local", "build_date": ""
+}
+
 warmup_cache()
 
 @app.before_request
@@ -101,7 +108,7 @@ def inject_render_time():
             "total": f"{total:.3f}",
             "breakdown": " | ".join(breakdown)
         }
-    return dict(get_render_time=get_render_time)
+    return dict(get_render_time=get_render_time, build_meta=BUILD_META)
 
 @app.route("/")
 def index():
@@ -113,6 +120,10 @@ def flush_cache_endpoint():
     deleted = flush_cache()
     return jsonify({"status": "ok", "deleted_files": deleted})
 
+@app.route("/version")
+def version():
+    return BUILD_META
+
 @app.route("/fees")
 def fees():
     attendance_url = f"https://docs.google.com/spreadsheets/d/{ATTENDANCE_SHEET_ID}/edit"