fix: switch sitemap/robots to dynamic generation + Nginx proxy

The reviewer noted the static sitemap.xml had a hardcoded domain in a
white-label app.  Instead of writing files to disk, we now serve
sitemap.xml and robots.txt as dynamic API endpoints that read
KMTN_WEBSITE_URL at runtime.

- Nginx proxies /sitemap.xml and /robots.txt to the backend API so
  search crawlers reach them at root paths.
- Removed the startup filesystem-write that would fail in Docker
  (backend can't write to Nginx container's static dir).
- Simplified sitemap_export.py to pure generation functions (no I/O).
- Added /api/sitemap/regenerate admin endpoint for consistency.

backend/app/main.py

@@ -183,14 +183,6 @@ async def lifespan(app: FastAPI):
     except Exception as e:
         print(f"  WARNING: Failed to write theme.json on startup: {e}")
 
-    # Write sitemap.xml + robots.txt on startup (tenant-aware)
-    try:
-        from app.sitemap_export import write_sitemap_files
-        info = write_sitemap_files()
-        print(f"  ✓ Sitemap + robots.txt written (base_url={info['base_url']})")
-    except Exception as e:
-        print(f"  WARNING: Failed to write sitemap on startup: {e}")
-
     # Check GeoLite2 database
     geo_db = settings.GEOLITE2_DB_PATH
     if os.path.exists(geo_db):