fix: Correct series filter logic for no_episodes

Critical bug fix: The filter was returning the wrong series because of
a misunderstanding of the episode table semantics.

ISSUE:
- Episodes table contains MISSING episodes (from episodeDict)
- is_downloaded=False means episode file not found in folder
- Original query logic was backwards - returned series with NO missing
  episodes instead of series WITH missing episodes

SOLUTION:
- Simplified query to directly check for episodes with is_downloaded=False
- Changed from complex join with count aggregation to simple subquery
- Now correctly returns series that have at least one undownloaded episode

CHANGES:
- src/server/database/service.py: Rewrote get_series_with_no_episodes()
  method with corrected logic and clearer documentation
- tests/unit/test_series_filter.py: Updated test expectations to match
  corrected behavior with detailed comments explaining episode semantics
- docs/API.md: Enhanced documentation explaining filter behavior and
  episode table meaning

TESTS:
All 5 unit tests pass with corrected logic

src/server/database/service.py

@@ -26,7 +26,7 @@ import logging
 from datetime import datetime, timedelta, timezone
 from typing import List, Optional
 
-from sqlalchemy import delete, select, update
+from sqlalchemy import Integer, delete, select, update
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.orm import Session, selectinload
 
@@ -258,11 +258,15 @@ class AnimeSeriesService:
         limit: Optional[int] = None,
         offset: int = 0,
     ) -> List[AnimeSeries]:
-        """Get anime series that have no downloaded episodes in folder.
+        """Get anime series that have no episodes found in folder.
         
-        Returns series where either:
-        - No episodes exist in the database, OR
-        - All episodes have is_downloaded=False
+        Since episodes in the database represent MISSING episodes
+        (from episodeDict), this returns series that have episodes
+        in the DB with is_downloaded=False, meaning they have missing
+        episodes and no files were found in the folder for those episodes.
+        
+        Returns series where:
+        - At least one episode exists in database with is_downloaded=False
         
         Args:
             db: Database session
@@ -270,31 +274,20 @@ class AnimeSeriesService:
             offset: Offset for pagination
         
         Returns:
-            List of AnimeSeries instances with no downloaded episodes
+            List of AnimeSeries with missing episodes (not in folder)
         """
-        from sqlalchemy import func, or_
-        
-        # Subquery to count downloaded episodes per series
-        downloaded_count = (
-            select(
-                Episode.series_id,
-                func.count(Episode.id).label('downloaded_count')
-            )
-            .where(Episode.is_downloaded.is_(True))
-            .group_by(Episode.series_id)
+        # Subquery to find series IDs with at least one undownloaded episode
+        undownloaded_series_ids = (
+            select(Episode.series_id)
+            .where(Episode.is_downloaded == False)
+            .distinct()
             .subquery()
         )
         
-        # Select series that either have no episodes or no downloaded episodes
+        # Select series that have undownloaded episodes
         query = (
             select(AnimeSeries)
-            .outerjoin(downloaded_count, AnimeSeries.id == downloaded_count.c.series_id)
-            .where(
-                or_(
-                    downloaded_count.c.downloaded_count == None,
-                    downloaded_count.c.downloaded_count == 0
-                )
-            )
+            .where(AnimeSeries.id.in_(select(undownloaded_series_ids.c.series_id)))
             .order_by(AnimeSeries.name)
             .offset(offset)
         )