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)

tests/unit/test_serie_class.py

@@ -173,6 +173,8 @@ class TestSerieProperties:
 
     def test_serie_save_and_load_from_file(self):
         """Test saving and loading Serie from file."""
+        import warnings
+        
         serie = Serie(
             key="test-key",
             name="Test Series",
@@ -190,11 +192,15 @@ class TestSerieProperties:
             temp_filename = f.name
         
         try:
-            # Save to file
-            serie.save_to_file(temp_filename)
-            
-            # Load from file
-            loaded_serie = Serie.load_from_file(temp_filename)
+            # Suppress deprecation warnings for this test
+            with warnings.catch_warnings():
+                warnings.simplefilter("ignore", DeprecationWarning)
+                
+                # Save to file
+                serie.save_to_file(temp_filename)
+                
+                # Load from file
+                loaded_serie = Serie.load_from_file(temp_filename)
             
             # Verify all properties match
             assert loaded_serie.key == serie.key
@@ -242,3 +248,75 @@ class TestSerieDocumentation:
         assert Serie.folder.fget.__doc__ is not None
         assert "metadata" in Serie.folder.fget.__doc__.lower()
         assert "not used for lookups" in Serie.folder.fget.__doc__.lower()
+
+
+class TestSerieDeprecationWarnings:
+    """Test deprecation warnings for file-based methods."""
+
+    def test_save_to_file_raises_deprecation_warning(self):
+        """Test save_to_file() raises deprecation warning."""
+        import warnings
+        
+        serie = Serie(
+            key="test-key",
+            name="Test Series",
+            site="https://example.com",
+            folder="Test Folder",
+            episodeDict={1: [1, 2, 3]}
+        )
+        
+        with tempfile.NamedTemporaryFile(
+            mode='w', suffix='.json', delete=False
+        ) as temp_file:
+            temp_filename = temp_file.name
+        
+        try:
+            with warnings.catch_warnings(record=True) as w:
+                warnings.simplefilter("always")
+                serie.save_to_file(temp_filename)
+                
+                # Check deprecation warning was raised
+                assert len(w) == 1
+                assert issubclass(w[0].category, DeprecationWarning)
+                assert "deprecated" in str(w[0].message).lower()
+                assert "save_to_file" in str(w[0].message)
+        finally:
+            if os.path.exists(temp_filename):
+                os.remove(temp_filename)
+
+    def test_load_from_file_raises_deprecation_warning(self):
+        """Test load_from_file() raises deprecation warning."""
+        import warnings
+        
+        serie = Serie(
+            key="test-key",
+            name="Test Series",
+            site="https://example.com",
+            folder="Test Folder",
+            episodeDict={1: [1, 2, 3]}
+        )
+        
+        with tempfile.NamedTemporaryFile(
+            mode='w', suffix='.json', delete=False
+        ) as temp_file:
+            temp_filename = temp_file.name
+        
+        try:
+            # Save first (suppress warning for this)
+            with warnings.catch_warnings():
+                warnings.simplefilter("ignore")
+                serie.save_to_file(temp_filename)
+            
+            # Now test loading
+            with warnings.catch_warnings(record=True) as w:
+                warnings.simplefilter("always")
+                Serie.load_from_file(temp_filename)
+                
+                # Check deprecation warning was raised
+                assert len(w) == 1
+                assert issubclass(w[0].category, DeprecationWarning)
+                assert "deprecated" in str(w[0].message).lower()
+                assert "load_from_file" in str(w[0].message)
+        finally:
+            if os.path.exists(temp_filename):
+                os.remove(temp_filename)