Fix stale data file updates on download completion

When episodes are downloaded successfully, the in-memory Serie.episodeDict
is updated, but the deprecated data file was not being synced. This caused
UI to show episodes as missing when already downloaded.

Changes:
- Update data file in _remove_episode_from_memory when download completes
- DB is authoritative; data file is optional backup (deprecated)
- Gracefully skip update if data file doesn't exist

New integration tests for episode download sync:
- Verify episode removed from missing list after download
- Verify in-memory cache updated after download
- Verify data file updated after download (when it exists)
- Verify downloads work without data file

docs/diagrams/system-architecture.mmd

@@ -31,14 +31,16 @@ flowchart TB
 
     subgraph Core["Core Layer"]
         SeriesApp["SeriesApp"]
+        SeriesCache["SeriesCache<br/>(In-Memory)"]
         SerieScanner["SerieScanner"]
         SerieList["SerieList"]
     end
 
     subgraph Data["Data Layer"]
-        SQLite[(SQLite<br/>aniworld.db)]
+        SQLite[("SQLite<br/>aniworld.db")]
         ConfigJSON[(config.json)]
-        FileSystem[(File System<br/>Anime Directory)]
+        FileSystem[(File System<br/>Anime Episodes)]
+        LegacyFiles[("Legacy Files<br/>key/data<br/>(Deprecated)")]
     end
 
     subgraph External["External"]
@@ -71,9 +73,13 @@ flowchart TB
     AnimeService --> SQLite
 
     %% Core to Data
+    SeriesApp --> SeriesCache
+    SeriesCache -.->|Cached Series| SQLite
     SeriesApp --> SerieScanner
     SeriesApp --> SerieList
-    SerieScanner --> FileSystem
+    SerieScanner -->|Scan Episodes| FileSystem
+    SerieScanner -->|Detect Series| SQLite
+    SerieScanner -->|Migrate Legacy| LegacyFiles
     SerieScanner --> Provider
 
     %% Event flow