refactor: Restructure downloaders with clear separation

This commit implements a complete reorganization of the downloader system
with a clear distinction between anime streaming sites and video hosting services.

## Structure Changes

**New Organization:**
- `app/downloaders/anime_sites/` - Anime streaming sites (catalogs + metadata)
- `app/downloaders/video_players/` - Video hosting services (file downloads)

**Base Classes:**
- `BaseAnimeSite` - For anime providers (search, episodes, metadata)
- `BaseVideoPlayer` - For video players (download link extraction)

**Migrated Downloaders:**
Anime Sites (4):
- AnimeSama, NekoSama, AnimeUltime, Vostfree

Video Players (8):
- Doodstream, Sibnet, VidMoly, SendVid, Lpayer, 1fichier, Uptobox, Rapidfile

## Key Improvements

1. **Clear Separation**: Distinct base classes for different use cases
2. **Preserved Functionality**: All existing features maintained
   - VidMoly: M3U8 support, Playwright, multi-domains, target_filename param
   - SendVid: target_filename parameter support
   - All others: No behavioral changes

3. **Better Organization**:
   - Anime sites: search_anime(), get_episodes(), get_anime_metadata()
   - Video players: get_download_link(url, target_filename=None)

4. **Fixed Imports**: Updated cross-imports in AnimeSama
   - from ..video_players.vidmoly import
   - from ..video_players.sendvid import
   - from ..video_players.sibnet import
   - from ..video_players.lpayer import

5. **Updated Tests**: All test imports use new structure
6. **Updated Providers**: Added 4 missing file hosts to providers.py

## Backward Compatibility

✅ Main API unchanged: get_downloader() works identically
✅ All 23 tests passing
✅ Frontend fully functional
✅ No breaking changes for users

## Documentation

- RESTRUCTURATION_SUMMARY.md - Technical details
- FIX_IMPORT_ERROR.md - Import error resolution
- IMPORT_VERIFICATION_REPORT.md - Complete import verification
- FRONTEND_VERIFICATION_FINAL.md - Frontend validation

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

app/downloaders/anime_sites/__init__.py

@@ -0,0 +1,32 @@
+"""Anime streaming sites (catalogs) downloaders"""
+from .base import BaseAnimeSite
+# Import all anime site downloaders
+from .animesama import AnimeSamaDownloader
+from .nekosama import NekoSamaDownloader
+from .animeultime import AnimeUltimeDownloader
+from .vostfree import VostfreeDownloader
+
+__all__ = [
+    "BaseAnimeSite",
+    "AnimeSamaDownloader",
+    "NekoSamaDownloader",
+    "AnimeUltimeDownloader",
+    "VostfreeDownloader",
+]
+
+
+def get_anime_site(url: str) -> BaseAnimeSite:
+    """Factory function to get the appropriate anime site for a URL"""
+    sites = [
+        AnimeSamaDownloader(),
+        AnimeUltimeDownloader(),
+        NekoSamaDownloader(),
+        VostfreeDownloader(),
+    ]
+
+    for site in sites:
+        if site.can_handle(url):
+            return site
+
+    # Return None if no match (should not happen in normal flow)
+    return None