Phase 8: Documentation and deprecation warnings for identifier standardization

- Enhanced infrastructure.md with identifier convention table, format requirements, migration notes
- Updated docs/README.md with series identifier convention section
- Updated docs/api_reference.md with key-based API examples and notes
- Added deprecation warnings to SerieList.get_by_folder()
- Added deprecation warnings to anime.py folder fallback lookup
- Added deprecation warnings to validate_series_key_or_folder()
- All warnings include v3.0.0 removal timeline
- All 1006 tests pass

src/core/entities/SerieList.py

@@ -2,6 +2,7 @@
 
 import logging
 import os
+import warnings
 from json import JSONDecodeError
 from typing import Dict, Iterable, List, Optional
 
@@ -144,6 +145,11 @@ class SerieList:
         """
         Get a series by its folder name.
         
+        .. deprecated:: 2.0.0
+            Use :meth:`get_by_key` instead. Folder-based lookups will be
+            removed in version 3.0.0. The `folder` field is metadata only
+            and should not be used for identification.
+        
         This method is provided for backward compatibility only.
         Prefer using get_by_key() for new code.
         
@@ -153,6 +159,12 @@ class SerieList:
         Returns:
             The Serie instance if found, None otherwise
         """
+        warnings.warn(
+            "get_by_folder() is deprecated and will be removed in v3.0.0. "
+            "Use get_by_key() instead. The 'folder' field is metadata only.",
+            DeprecationWarning,
+            stacklevel=2
+        )
         for serie in self.keyDict.values():
             if serie.folder == folder:
                 return serie