Apply final-review cleanup: robots sitemap, CI guards, header parity

- Remove Sitemap line from robots.txt (no @astrojs/sitemap installed; was
  pointing to a 404 that would trip Google Search Console).
- Align Permissions-Policy across all three enforcement layers (middleware,
  .htaccess, Cloudflare Transform Rule in OPERATOR-CHECKLIST) by dropping the
  stray fullscreen=(self) from the middleware.
- Bump Lighthouse CI numberOfRuns from 1 to 3 to dampen CI-runner noise.
- Add CI guard that fails the build if any <TBD:...> marker survives into
  dist/ — prevents a legally incomplete imprint from shipping by accident.
- Add SFTP_* secret null-guard before the rsync --delete step so a missing
  secret fails loudly instead of targeting the SSH user's home root.
- Document the set:html compile-time-constant invariant in DualValueProps.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/deploy.yml

@@ -31,6 +31,14 @@ jobs:
       - name: Build site
         run: npm run build
 
+      - name: Guard — no TBD markers may ship in built HTML
+        run: |
+          if grep -rl 'TBD:' dist 2>/dev/null | grep -E '\.(html|svg)$'; then
+            echo "Built output contains unfilled <TBD:...> markers."
+            echo "Fill in imprint.astro and privacy.astro operator fields before merging to main."
+            exit 1
+          fi
+
       - name: Validate HTML
         run: npm run lint:html
 
@@ -80,6 +88,11 @@ jobs:
           SFTP_HOST: ${{ secrets.SFTP_HOST }}
           SFTP_PATH: ${{ secrets.SFTP_PATH }}
         run: |
+          # Fail loudly if any secret is missing — otherwise rsync --delete
+          # could be directed at the SSH user's home root.
+          : "${SFTP_USER:?SFTP_USER secret must be set}"
+          : "${SFTP_HOST:?SFTP_HOST secret must be set}"
+          : "${SFTP_PATH:?SFTP_PATH secret must be set}"
           rsync -avz --delete \
             -e "ssh -i ~/.ssh/id_ed25519 -o StrictHostKeyChecking=yes -o UserKnownHostsFile=~/.ssh/known_hosts" \
             dist/ "$SFTP_USER@$SFTP_HOST:$SFTP_PATH/"