fix: exclude downloaded episodes from episodeDict and GetMissingEpisode

Prevent fully-downloaded series from being queued by auto-download:

- SerieList.GetMissingEpisode(): filter by is_downloaded instead of
  checking if episodeDict is non-empty. episodeDict from the DB
  relationship includes all episodes (including downloaded), so a
  series with only downloaded episodes still had a truthy episodeDict.

- AnimeSeries.episodeDict property: skip episodes where is_downloaded=True
  when building the dict from the DB relationship. This makes the
  property consistent with the is_downloaded filtering already done
  manually in list_series_with_filters(), and ensures that calling
  code anywhere in the codebase gets the correct missing-episode view.

Fixes hana-kimi (and any other fully-downloaded series) incorrectly
appearing in the auto-download queue after a rescan.

src/server/database/models.py

@@ -205,6 +205,8 @@ class AnimeSeries(Base, TimestampMixin):
         try:
             if self.episodes:
                 for ep in self.episodes:
+                    if ep.is_downloaded:
+                        continue
                     season = ep.season or 1
                     if season not in episode_dict:
                         episode_dict[season] = []