feat: Add SendVid downloader support

Add complete support for SendVid video hosting service used by Anime-Sama
for anime series like Hell's Paradise.

Changes:
- Create SendVidDownloader class with proper headers to avoid 403 errors
- Add SendVid detection and handling in AnimeSamaDownloader
- Update download_manager to include SendVid-specific headers
- Support custom episode naming (e.g., "Hells Paradise - Episode 01.mp4")

Technical details:
- SendVid embed pages require User-Agent and Referer headers
- Direct MP4 URLs extracted from <source> tags with IP/time-based parameters
- Tested with Hell's Paradise Episode 01 (7MB, 24min, 1280x720)

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/base.py

@@ -0,0 +1,54 @@
+from abc import ABC, abstractmethod
+from typing import Optional, Tuple
+import httpx
+import re
+from bs4 import BeautifulSoup
+
+
+class BaseDownloader(ABC):
+    """Base class for all host downloaders"""
+
+    def __init__(self):
+        self.client = httpx.AsyncClient(timeout=10.0, follow_redirects=True)
+
+    @abstractmethod
+    async def get_download_link(self, url: str) -> Tuple[str, str]:
+        """
+        Extract direct download link and filename from host URL
+        Returns: (download_url, filename)
+        """
+        pass
+
+    @abstractmethod
+    def can_handle(self, url: str) -> bool:
+        """Check if this downloader can handle the given URL"""
+        pass
+
+    async def close(self):
+        await self.client.aclose()
+
+    async def _fetch_page(self, url: str) -> str:
+        response = await self.client.get(url)
+        response.raise_for_status()
+        return response.text
+
+    def _extract_filename_from_headers(self, headers: dict) -> Optional[str]:
+        content_disposition = headers.get("content-disposition", "")
+        if "filename=" in content_disposition:
+            filename = content_disposition.split("filename=")[-1].strip('"')
+            return filename
+        return None
+
+    async def search_anime(self, query: str, lang: str = "vostfr") -> list[dict]:
+        """
+        Search for anime on this provider
+        Returns list of anime with title, url, and optional cover image
+        """
+        return []
+
+    async def get_episodes(self, anime_url: str, lang: str = "vostfr") -> list[dict]:
+        """
+        Get list of episodes for an anime
+        Returns list of episode numbers and their URLs
+        """
+        return []