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/api/anime.py

@@ -337,10 +337,8 @@ async def list_anime(
             # Get all series from database to fetch NFO metadata
             # and episode counts
             from src.server.database.connection import get_sync_session
-            from src.server.database.models import (
-                AnimeSeries as DBAnimeSeries,
-                Episode
-            )
+            from src.server.database.models import AnimeSeries as DBAnimeSeries
+            from src.server.database.models import Episode
             
             session = get_sync_session()
             try: