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/sitemap_export.py

@@ -1,24 +1,13 @@
-"""Sitemap & robots.txt export: generate tenant-aware SEO files.
+"""Sitemap & robots.txt generation: tenant-aware SEO content.
 
 Because this is a white-label app where each tenant has its own domain
 (set via KMTN_WEBSITE_URL), the sitemap and robots.txt are generated
-at runtime from the station config — one static write per startup,
-plus an optional admin-triggered regeneration.
-
-Follows the same pattern as theme_export.py:
-- Atomic write (tmp + os.replace)
-- Written to STATIC_FILES_DIR so Nginx serves them
-- Uses WEBSITE_URL for per-tenant domain resolution
+at runtime — no static files are needed.  The FastAPI /api/sitemap/*
+endpoints call these functions; Nginx proxies /sitemap.xml and
+/robots.txt to those endpoints so search-engine crawlers hit the right
+content at the root URL.
 """
 
-import logging
-from datetime import datetime, timezone
-from pathlib import Path
-
-from app.config import settings
-
-logger = logging.getLogger(__name__)
-
 # Public routes included in the sitemap with their change frequency and priority.
 SEOMAP_ROUTES = [
     {"path": "/", "changefreq": "daily", "priority": 1.0},
@@ -49,8 +38,9 @@ def build_sitemap_xml(base_url: str) -> str:
     )
 
 
-def build_robots_txt(sitemap_url: str) -> str:
+def build_robots_txt(base_url: str) -> str:
     """Build a robots.txt string that references the correct sitemap URL."""
+    sitemap = base_url.rstrip("/") + "/sitemap.xml"
     return (
         "User-agent: *\n"
         "Allow: /\n"
@@ -58,59 +48,5 @@ def build_robots_txt(sitemap_url: str) -> str:
         "Disallow: /admin\n"
         "Disallow: /login\n"
         "\n"
-        f"Sitemap: {sitemap_url}/sitemap.xml\n"
+        f"Sitemap: {sitemap}\n"
     )
-
-
-def get_sitemap_status() -> dict:
-    """Check if sitemap.xml and robots.txt exist on disk and return metadata."""
-    base = Path(settings.STATIC_FILES_DIR)
-    result = {}
-    for name in ("sitemap.xml", "robots.txt"):
-        path = base / name
-        if not path.exists():
-            result[name] = {"exists": False, "path": str(path)}
-            continue
-        stat = path.stat()
-        result[name] = {
-            "exists": True,
-            "path": str(path),
-            "size": stat.st_size,
-            "modified": datetime.fromtimestamp(stat.st_mtime, tz=timezone.utc).isoformat(),
-        }
-    return result
-
-
-def write_sitemap_files() -> dict:
-    """Write sitemap.xml and robots.txt to the static files directory.
-
-    Uses atomic write (write to .tmp, then os.replace) to prevent
-    Nginx from serving a partially-written file.
-
-    Returns a status dict with file paths.
-    """
-    base_url = settings.WEBSITE_URL
-    static_dir = Path(settings.STATIC_FILES_DIR)
-    static_dir.mkdir(parents=True, exist_ok=True)
-
-    # sitemap.xml
-    sitemap_content = build_sitemap_xml(base_url)
-    sitemap_path = static_dir / "sitemap.xml"
-    tmp_path = sitemap_path.with_suffix(".tmp")
-    tmp_path.write_text(sitemap_content, encoding="utf-8")
-    tmp_path.replace(sitemap_path)
-    logger.info("Wrote sitemap.xml (%d bytes) to %s, base_url=%s", len(sitemap_content), sitemap_path, base_url)
-
-    # robots.txt
-    robots_content = build_robots_txt(base_url)
-    robots_path = static_dir / "robots.txt"
-    tmp_path = robots_path.with_suffix(".tmp")
-    tmp_path.write_text(robots_content, encoding="utf-8")
-    tmp_path.replace(robots_path)
-    logger.info("Wrote robots.txt (%d bytes) to %s", len(robots_content), robots_path)
-
-    return {
-        "sitemap.xml": str(sitemap_path),
-        "robots.txt": str(robots_path),
-        "base_url": base_url,
-    }