feat(merchant): extract merchant portal as first-class frontend with auth, Tailwind fixes, and Gitea CI

- Extract login/dashboard from billing module into core (matching admin pattern)
- Add merchant auth API with path-isolated cookies (path=/merchants)
- Add merchant base layout with sidebar/header partials and Alpine.js init
- Add frontend detection and login redirect for MERCHANT type
- Wire merchant token in shared api-client.js (get/clear)
- Migrate billing templates to merchant base with dark mode support
- Fix Tailwind: rename shop→storefront in sources and config
- DRY Makefile tailwind targets with TAILWIND_FRONTENDS loop
- Rebuild all Tailwind outputs (production minified)
- Add Gitea Actions CI workflow (ruff, pytest, architecture, docs)
- Add Gitea deployment documentation

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

app/exceptions/handler.py

@@ -393,6 +393,9 @@ def _redirect_to_login(request: Request) -> RedirectResponse:
     if frontend_type == FrontendType.ADMIN:
         logger.debug("Redirecting to /admin/login")
         return RedirectResponse(url="/admin/login", status_code=302)
+    if frontend_type == FrontendType.MERCHANT:
+        logger.debug("Redirecting to /merchants/login")
+        return RedirectResponse(url="/merchants/login", status_code=302)
     if frontend_type == FrontendType.STORE:
         # Extract store code from the request path
         # Path format: /store/{store_code}/...