Use the git commit hash as the app version

- Spec header, status pill and footer now document '<env> @ <hash>'
  instead of invented release numbers
- Backend settings expose GIT_COMMIT_HASH / APP_ENV / APP_VERSION,
  mirroring the original app
- Frontend bakes the short hash in at build time and shows it in the
  top bar

backend/config/settings.py

@@ -3,6 +3,7 @@ Django settings for the J621 backend.
 """
 
 import os
+import subprocess
 from pathlib import Path
 
 from dotenv import load_dotenv
@@ -19,6 +20,24 @@ ALLOWED_HOSTS = [
     if host.strip()
 ]
 
+
+def _git_commit_hash():
+    """Short git hash of the running build, mirroring the original app."""
+    try:
+        return subprocess.check_output(
+            ["git", "rev-parse", "--short", "HEAD"],
+            cwd=BASE_DIR,
+            text=True,
+            stderr=subprocess.DEVNULL,
+        ).strip()
+    except (subprocess.SubprocessError, OSError):
+        return "unknown"
+
+
+GIT_COMMIT_HASH = _git_commit_hash()
+APP_ENV = "dev" if DEBUG else "prod"
+APP_VERSION = f"{APP_ENV} @ {GIT_COMMIT_HASH}"
+
 # Application definition
 
 INSTALLED_APPS = [