chore: Add deprecation warnings and update documentation (Task 9)

Task 9: Clean up legacy code
- Added deprecation warnings to Serie.save_to_file() and load_from_file()
- Updated infrastructure.md with Data Storage section documenting:
  - SQLite database as primary storage
  - Legacy file storage as deprecated
  - Data migration process
- Added deprecation warning tests for Serie class
- Updated existing tests to handle new warnings
- All 1012 tests pass (872 unit + 55 API + 85 integration)

src/core/entities/series.py

@@ -1,4 +1,5 @@
 import json
+import warnings
 
 
 class Serie:
@@ -154,13 +155,46 @@ class Serie:
         )
 
     def save_to_file(self, filename: str):
-        """Save Serie object to JSON file."""
+        """Save Serie object to JSON file.
+        
+        .. deprecated::
+            File-based storage is deprecated. Use database storage via
+            `AnimeSeriesService.create()` instead. This method will be
+            removed in v3.0.0.
+            
+        Args:
+            filename: Path to save the JSON file
+        """
+        warnings.warn(
+            "save_to_file() is deprecated and will be removed in v3.0.0. "
+            "Use database storage via AnimeSeriesService.create() instead.",
+            DeprecationWarning,
+            stacklevel=2
+        )
         with open(filename, "w", encoding="utf-8") as file:
             json.dump(self.to_dict(), file, indent=4)
 
     @classmethod
     def load_from_file(cls, filename: str) -> "Serie":
-        """Load Serie object from JSON file."""
+        """Load Serie object from JSON file.
+        
+        .. deprecated::
+            File-based storage is deprecated. Use database storage via
+            `AnimeSeriesService.get_by_key()` instead. This method will be
+            removed in v3.0.0.
+            
+        Args:
+            filename: Path to load the JSON file from
+            
+        Returns:
+            Serie: The loaded Serie object
+        """
+        warnings.warn(
+            "load_from_file() is deprecated and will be removed in v3.0.0. "
+            "Use database storage via AnimeSeriesService instead.",
+            DeprecationWarning,
+            stacklevel=2
+        )
         with open(filename, "r", encoding="utf-8") as file:
             data = json.load(file)
         return cls.from_dict(data)