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_list.py

@@ -473,10 +473,19 @@ class TestSerieListDeprecationWarnings:
             warnings.simplefilter("always")
             serie_list.add(sample_serie)
             
-            # Check deprecation warning was raised
-            assert len(w) == 1
-            assert issubclass(w[0].category, DeprecationWarning)
-            assert "add_to_db()" in str(w[0].message)
+            # Check at least one deprecation warning was raised for add()
+            # (Note: save_to_file also raises a warning, so we may get 2)
+            deprecation_warnings = [
+                warning for warning in w
+                if issubclass(warning.category, DeprecationWarning)
+            ]
+            assert len(deprecation_warnings) >= 1
+            # Check that one of them is from add()
+            add_warnings = [
+                warning for warning in deprecation_warnings
+                if "add_to_db()" in str(warning.message)
+            ]
+            assert len(add_warnings) == 1
 
     def test_get_by_folder_raises_deprecation_warning(
         self, temp_directory, sample_serie