feat: add English fallback for empty German TMDB overview in NFO creation

When TMDB returns an empty German (de-DE) overview for anime (e.g.
Basilisk), the NFO plot tag was missing. Now both create and update
paths call _enrich_details_with_fallback() which fetches the English
(en-US) overview as a fallback.

Additionally, the <plot> XML element is always written (even when
empty) via the always_write parameter on _add_element(), ensuring
consistent NFO structure regardless of creation path.

Changes:
- nfo_service.py: add _enrich_details_with_fallback() method, call it
  in create_tvshow_nfo and update_tvshow_nfo
- nfo_generator.py: add always_write param to _add_element(), use it
  for <plot> tag
- test_nfo_service.py: add TestEnrichDetailsWithFallback with 4 tests

src/core/services/nfo_service.py

@@ -169,6 +169,9 @@ class NFOService:
             # Get content ratings for FSK
             content_ratings = await self.tmdb_client.get_tv_show_content_ratings(tv_id)
             
+            # Enrich with English fallback for empty overview/tagline
+            details = await self._enrich_details_with_fallback(details)
+            
             # Convert TMDB data to TVShowNFO model
             nfo_model = tmdb_to_nfo_model(
                 details,
@@ -264,6 +267,9 @@ class NFOService:
             # Get content ratings for FSK
             content_ratings = await self.tmdb_client.get_tv_show_content_ratings(tmdb_id)
             
+            # Enrich with English fallback for empty overview/tagline
+            details = await self._enrich_details_with_fallback(details)
+            
             # Convert TMDB data to TVShowNFO model
             nfo_model = tmdb_to_nfo_model(
                 details,
@@ -372,6 +378,61 @@ class NFOService:
         
         return result
     
+    async def _enrich_details_with_fallback(
+        self,
+        details: Dict[str, Any],
+    ) -> Dict[str, Any]:
+        """Enrich TMDB details with English fallback for empty fields.
+
+        When requesting details in ``de-DE``, some anime have an empty
+        ``overview`` (and potentially other translatable fields).  This
+        method detects empty values and fills them from the English
+        (``en-US``) endpoint so that NFO files always contain a ``plot``
+        regardless of whether the German translation exists.
+
+        Args:
+            details: TMDB TV show details (language ``de-DE``).
+
+        Returns:
+            The *same* dict, mutated in-place with English fallbacks
+            where needed.
+        """
+        overview = details.get("overview") or ""
+
+        if overview:
+            # Overview already populated – nothing to do.
+            return details
+
+        logger.debug(
+            "German overview empty for TMDB ID %s, fetching English fallback",
+            details.get("id"),
+        )
+
+        try:
+            en_details = await self.tmdb_client.get_tv_show_details(
+                details["id"],
+                language="en-US",
+            )
+
+            if en_details.get("overview"):
+                details["overview"] = en_details["overview"]
+                logger.info(
+                    "Used English overview fallback for TMDB ID %s",
+                    details.get("id"),
+                )
+
+            # Also fill tagline if missing
+            if not details.get("tagline") and en_details.get("tagline"):
+                details["tagline"] = en_details["tagline"]
+        except Exception as exc:  # pylint: disable=broad-except
+            logger.warning(
+                "Failed to fetch English fallback for TMDB ID %s: %s",
+                details.get("id"),
+                exc,
+            )
+
+        return details
+
     def _find_best_match(
         self,
         results: List[Dict[str, Any]],