refactor(download): mark episode downloaded instead of deleting

Change _remove_episode_from_missing_list to set is_downloaded=True
and populate file_path via EpisodeService.mark_downloaded, instead of
deleting the Episode row. Preserves download history so queries can
distinguish series with downloaded episodes from completely unwatched
series.

- Pass serie_folder to construct file_path
- Look up series_id via AnimeSeriesService.get_by_key
- Update tests to mock mark_downloaded path
- Document episode lifecycle in docs/DEVELOPMENT.md

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

tests/unit/test_download_service.py

@@ -79,6 +79,7 @@ def mock_anime_service():
     """Create a mock AnimeService."""
     service = MagicMock(spec=AnimeService)
     service.download = AsyncMock(return_value=True)
+    service._directory = "/mock/anime/directory"
     return service
 
 
@@ -731,13 +732,22 @@ class TestRemoveEpisodeFromMissingList:
         download_service._anime_service._app = mock_app
         download_service._anime_service._cached_list_missing = MagicMock()
 
-        # Mock DB call
+        # Mock DB session
         mock_db_session = AsyncMock()
-        mock_delete = AsyncMock(return_value=True)
+        
+        # Mock series returned by get_by_key
+        mock_series = MagicMock()
+        mock_series.id = 1
+        
+        # Mock episode returned by get_by_episode
+        mock_episode = MagicMock()
+        mock_episode.id = 100
 
         with patch(
             "src.server.database.connection.get_db_session"
         ) as mock_get_db, patch(
+            "src.server.database.service.AnimeSeriesService"
+        ) as mock_series_svc, patch(
             "src.server.database.service.EpisodeService"
         ) as mock_ep_svc:
             mock_get_db.return_value.__aenter__ = AsyncMock(
@@ -746,20 +756,30 @@ class TestRemoveEpisodeFromMissingList:
             mock_get_db.return_value.__aexit__ = AsyncMock(
                 return_value=False
             )
-            mock_ep_svc.delete_by_series_and_episode = mock_delete
+            
+            # Mock get_by_key to return series
+            mock_series_svc.get_by_key = AsyncMock(return_value=mock_series)
+            
+            # Mock get_by_episode to return episode
+            mock_ep_svc.get_by_episode = AsyncMock(return_value=mock_episode)
+            
+            # Mock mark_downloaded to succeed
+            mock_ep_svc.mark_downloaded = AsyncMock(return_value=mock_episode)
 
             result = await download_service._remove_episode_from_missing_list(
                 series_key="test-series",
                 season=1,
                 episode=2,
+                serie_folder="Test Series (2024)",
             )
 
-        # DB deletion was called
-        mock_delete.assert_awaited_once_with(
+        # mark_downloaded was called instead of delete
+        mock_ep_svc.mark_downloaded.assert_awaited_once_with(
             db=mock_db_session,
-            series_key="test-series",
-            season=1,
-            episode_number=2,
+            episode_id=100,
+            file_path=(
+                f"{download_service._directory}/Test Series (2024)/Season 1"
+            ),
         )
         # In-memory update happened
         assert 2 not in serie.episodeDict[1]
@@ -807,11 +827,20 @@ class TestRemoveEpisodeFromMissingList:
 
         # Mock DB calls
         mock_db_session = AsyncMock()
-        mock_delete = AsyncMock(return_value=True)
+        
+        # Mock series returned by get_by_key
+        mock_series = MagicMock()
+        mock_series.id = 1
+        
+        # Mock episode returned by get_by_episode
+        mock_episode = MagicMock()
+        mock_episode.id = 100
 
         with patch(
             "src.server.database.connection.get_db_session"
         ) as mock_get_db, patch(
+            "src.server.database.service.AnimeSeriesService"
+        ) as mock_series_svc, patch(
             "src.server.database.service.EpisodeService"
         ) as mock_ep_svc:
             mock_get_db.return_value.__aenter__ = AsyncMock(
@@ -820,7 +849,15 @@ class TestRemoveEpisodeFromMissingList:
             mock_get_db.return_value.__aexit__ = AsyncMock(
                 return_value=False
             )
-            mock_ep_svc.delete_by_series_and_episode = mock_delete
+            
+            # Mock get_by_key to return series
+            mock_series_svc.get_by_key = AsyncMock(return_value=mock_series)
+            
+            # Mock get_by_episode to return episode
+            mock_ep_svc.get_by_episode = AsyncMock(return_value=mock_episode)
+            
+            # Mock mark_downloaded to succeed
+            mock_ep_svc.mark_downloaded = AsyncMock(return_value=mock_episode)
 
             # Process the download
             item = download_service._pending_queue.popleft()