fix: address reviewer feedback — dynamic sitemap/robots + wildcard CI branches

- CI workflow: replace hardcoded branch list with ['**'] wildcard so
  pipeline runs on every branch push, not just 4 named branches.

- Sitemap/robots.txt: moved from static files to dynamic generation.
  New sitemap_export.py module (follows theme_export.py pattern) uses
  KMTN_WEBSITE_URL to generate tenant-aware sitemap.xml and robots.txt
  on app startup. Atomic writes to STATIC_FILES_DIR so Nginx serves them.

- Added /api/sitemap endpoint with /status (public) and /regenerate
  (admin-only) for runtime control.

Addresses reviewer comments on PR #4:
  1. 'more general way to ensure this runs on any branch on push'
  2. 'whitelabel app - this file may need to be dynamically generated'

.gitea/workflows/ci.yaml

@@ -2,7 +2,7 @@ name: CI Pipeline
 
 on:
   push:
-    branches: [main, develop, feature/tests-and-ci, feature/multi-admin-roles, fix/seo-improvements]
+    branches: ['**']
   pull_request:
     branches: [main]