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/MIGRATION_GUIDE.md

@@ -0,0 +1,111 @@
+# Migration Guide: File-Based to Database Storage
+
+## Overview
+
+This guide covers the transition from file-based series metadata storage to the new database-backed system introduced in v2.0.
+
+## What Changed
+
+**Before v2.0**: Series metadata stored in `key` and `data` files alongside anime folders.
+
+**After v2.0**: All metadata stored in SQLite database (`aniworld.db`). Files are deprecated but still supported for backward compatibility during migration.
+
+## Automated Migration
+
+The application automatically migrates on first startup:
+
+1. Scans anime directory for `key` and `data` files
+2. Parses legacy files into `AnimeSeries` and `Episode` records
+3. Loads series into in-memory cache
+4. Logs migration results
+
+**No manual action required.**
+
+## Manual Verification
+
+After first startup with the new version:
+
+1. **Check logs** for: `"Migrated X series from files to DB"`
+2. **Verify series count**: UI shows same number of series as before
+3. **Confirm episodes**: Episode counts match expected totals
+
+```bash
+# Check migration log
+grep "Migrated" logs/app.log
+
+# Verify series via API
+curl http://localhost:8000/api/anime | jq '.total'
+```
+
+## After Migration
+
+### Safe to Delete
+
+Once verified, these files can be removed:
+
+```
+<anime_folder>/
+├── Attack on Titan (2013)/
+│   ├── key     # ❌ Can delete
+│   ├── data    # ❌ Can delete
+│   └── Season 1/
+│       └── ...
+```
+
+**Deleting these files does not affect the database.** The metadata now lives in `aniworld.db`.
+
+### Backup (Recommended)
+
+Before deleting, backup the files:
+
+```bash
+# Create backup directory
+mkdir -p backup/legacy_series_files
+
+# Copy all key and data files
+find /path/to/anime -name "key" -o -name "data" | while read f; do
+    cp "$f" "backup/legacy_series_files/"
+done
+```
+
+## Reverting (Not Recommended)
+
+If you must revert to file-based storage:
+
+1. **Restore from database backup** (if available)
+2. **Export manually** (no export script exists)
+
+**Warning**: File-based storage is deprecated and will be removed in v3.0.0.
+
+## Troubleshooting
+
+### Series Not Appearing After Migration
+
+1. Check logs for migration errors: `grep -i error logs/app.log`
+2. Verify `key` and `data` files exist and are readable
+3. Manually trigger rescan: `POST /api/scheduler/trigger-rescan`
+
+### Duplicate Series
+
+1. Check for duplicate `key` files (same series in multiple folders)
+2. Verify series key uniqueness in database:
+
+```bash
+sqlite3 aniworld.db "SELECT key, COUNT(*) FROM anime_series GROUP BY key HAVING COUNT(*) > 1;"
+```
+
+### Missing Episodes
+
+1. Trigger targeted scan for affected series
+2. Check episode sync logs
+3. Verify file permissions on anime directory
+
+## Deprecation Timeline
+
+| Version | Status |
+|---------|--------|
+| v2.0.x  | Legacy files supported, migration automated |
+| v2.1.x  | Legacy files still supported, warnings in logs |
+| v3.0.0  | **Legacy files removed** - database only |
+
+Upgrade to v3.0.0 before legacy file support ends.