fix(ci): build release payload via heredoc, drop broken env-var passing

Previous attempt used `python3 -c "..." KEY=VALUE` which passes
KEY=VALUE as positional args, not environment variables — the python
block then crashed with KeyError: 'BODY' because nothing actually set
it in the environment.

Consolidate into a single heredoc-fed python3 block that reads
RELEASE_NOTES from the already-exported env var and reads TAG/VERSION/
IS_PRE after an explicit `export`. Uses <<'PY' so shell metachars in
the Python source (backticks, $, quotes) are not interpreted.

Also drops the redundant intermediate BODY variable — body is built
directly inside the single python invocation.

.gitea/workflows/release.yml

@@ -94,7 +94,9 @@ jobs:
             echo "No RELEASE_NOTES.md found"
           fi
 
-          BODY=$(python3 -c "
+          # Build release payload (avoids shell escaping & CLI length limits)
+          export TAG VERSION IS_PRE
+          python3 <<'PY'
           import json, os
 
           release_notes = os.environ.get('RELEASE_NOTES', '')
@@ -106,22 +108,16 @@ jobs:
           if changelog:
               sections.append('## Changelog\n\n' + changelog)
 
-          print(json.dumps('\n\n'.join(sections)))
-          ")
-
-          # Send body via file to avoid CLI length limits / shell escaping
-          python3 -c "
-          import json, os
-          body = os.environ['BODY']
           payload = {
               'tag_name': os.environ['TAG'],
-              'name': f\"Notify Bridge {os.environ['VERSION']}\",
-              'body': json.loads(body),
+              'name': f"Notify Bridge {os.environ['VERSION']}",
+              'body': '\n\n'.join(sections),
               'draft': False,
               'prerelease': os.environ['IS_PRE'] == 'true',
           }
-          open('/tmp/release-payload.json','w').write(json.dumps(payload))
-          " BODY="$BODY" TAG="$TAG" VERSION="$VERSION" IS_PRE="$IS_PRE"
+          with open('/tmp/release-payload.json', 'w') as f:
+              json.dump(payload, f)
+          PY
 
           HTTP=$(curl -s -o /tmp/release-resp.json -w "%{http_code}" \
             -X POST "$BASE_URL/releases" \