refactor: restructure core→server, split large entity files into database module

- Move src/core/ → src/server/
- Split SerieList.py (531 lines) and series.py (414 lines) into src/server/database/
- Add database/models.py for SQLAlchemy models
- Update all test imports to reflect new structure
- Remove deprecated test files (test_serie_class.py, test_serie_folder_with_year.py)

src/server/providers/base_provider.py

@@ -0,0 +1,104 @@
+from abc import ABC, abstractmethod
+from typing import Any, Dict, List
+
+
+class Loader(ABC):
+    """Abstract base class for anime data loaders/providers."""
+    @abstractmethod
+    def subscribe_download_progress(self, handler):
+        """Subscribe a handler to the download_progress event.
+        Args:
+            handler: Callable to be called with progress dict.
+        """
+    @abstractmethod
+    def unsubscribe_download_progress(self, handler):
+        """Unsubscribe a handler from the download_progress event.
+        Args:
+            handler: Callable previously subscribed.
+        """
+
+    @abstractmethod
+    def search(self, word: str) -> List[Dict[str, Any]]:
+        """Search for anime series by name.
+
+        Args:
+            word: Search term to look for
+
+        Returns:
+            List of found series as dictionaries containing series information
+        """
+
+    @abstractmethod
+    def is_language(
+        self,
+        season: int,
+        episode: int,
+        key: str,
+        language: str = "German Dub",
+    ) -> bool:
+        """Check if episode exists in specified language.
+
+        Args:
+            season: Season number (1-indexed)
+            episode: Episode number (1-indexed)
+            key: Unique series identifier/key
+            language: Language to check (default: German Dub)
+
+        Returns:
+            True if episode exists in specified language, False otherwise
+        """
+
+    @abstractmethod
+    def download(
+        self,
+        base_directory: str,
+        serie_folder: str,
+        season: int,
+        episode: int,
+        key: str,
+        language: str = "German Dub"
+    ) -> bool:
+        """Download episode to specified directory.
+
+        Args:
+            base_directory: Base directory for downloads
+            serie_folder: Series folder name within base directory
+            season: Season number (0 for movies, 1+ for series)
+            episode: Episode number within season
+            key: Unique series identifier/key
+            language: Language version to download (default: German Dub)
+
+        Returns:
+            True if download successful, False otherwise
+        """
+
+    @abstractmethod
+    def get_site_key(self) -> str:
+        """Get the site key/identifier for this provider.
+
+        Returns:
+            Site key string (e.g., 'aniworld.to', 'voe.com')
+        """
+
+    @abstractmethod
+    def get_title(self, key: str) -> str:
+        """Get the human-readable title of a series.
+
+        Args:
+            key: Unique series identifier/key
+
+        Returns:
+            Series title string
+        """
+
+    @abstractmethod
+    def get_season_episode_count(self, slug: str) -> Dict[int, int]:
+        """Get season and episode counts for a series.
+
+        Args:
+            slug: Series slug/key identifier
+
+        Returns:
+            Dictionary mapping season number (int) to episode count (int)
+        """
+