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/utils/nfo_generator.py

@@ -43,8 +43,10 @@ def generate_tvshow_nfo(tvshow: TVShowNFO, pretty_print: bool = True) -> str:
     _add_element(root, "sorttitle", tvshow.sorttitle)
     _add_element(root, "year", str(tvshow.year) if tvshow.year else None)
     
-    # Plot and description
-    _add_element(root, "plot", tvshow.plot)
+    # Plot and description – always write <plot> even when empty so that
+    # all NFO files have a consistent set of tags regardless of whether they
+    # were produced by create or update.
+    _add_element(root, "plot", tvshow.plot, always_write=True)
     _add_element(root, "outline", tvshow.outline)
     _add_element(root, "tagline", tvshow.tagline)
     
@@ -164,13 +166,23 @@ def generate_tvshow_nfo(tvshow: TVShowNFO, pretty_print: bool = True) -> str:
     return xml_declaration + xml_str
 
 
-def _add_element(parent: etree.Element, tag: str, text: Optional[str]) -> Optional[etree.Element]:
+def _add_element(
+    parent: etree.Element,
+    tag: str,
+    text: Optional[str],
+    always_write: bool = False,
+) -> Optional[etree.Element]:
     """Add a child element to parent if text is not None or empty.
     
     Args:
         parent: Parent XML element
         tag: Tag name for child element
-        text: Text content (None or empty strings are skipped)
+        text: Text content (None or empty strings are skipped
+              unless *always_write* is True)
+        always_write: When True the element is created even when
+                      *text* is None/empty (the element will have
+                      no text content).  Useful for tags like
+                      ``<plot>`` that should always be present.
         
     Returns:
         Created element or None if skipped
@@ -179,6 +191,8 @@ def _add_element(parent: etree.Element, tag: str, text: Optional[str]) -> Option
         elem = etree.SubElement(parent, tag)
         elem.text = text
         return elem
+    if always_write:
+        return etree.SubElement(parent, tag)
     return None