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/server/api/anime.py

@@ -1,3 +1,5 @@
+import logging
+import warnings
 from typing import Any, List, Optional
 
 from fastapi import APIRouter, Depends, HTTPException, status
@@ -11,6 +13,8 @@ from src.server.utils.dependencies import (
     require_auth,
 )
 
+logger = logging.getLogger(__name__)
+
 router = APIRouter(prefix="/api/anime", tags=["anime"])
 
 
@@ -715,6 +719,21 @@ async def get_anime(
             for serie in series:
                 if getattr(serie, "folder", None) == anime_id:
                     found = serie
+                    # Log deprecation warning for folder-based lookup
+                    key = getattr(serie, "key", "unknown")
+                    logger.warning(
+                        "Folder-based lookup for '%s' is deprecated. "
+                        "Use series key '%s' instead. Folder-based lookups "
+                        "will be removed in v3.0.0.",
+                        anime_id,
+                        key
+                    )
+                    warnings.warn(
+                        f"Folder-based lookup for '{anime_id}' is deprecated. "
+                        f"Use series key '{key}' instead.",
+                        DeprecationWarning,
+                        stacklevel=2
+                    )
                     break
 
         if not found: