Phase 6: Update database layer identifier documentation

- Updated AnimeSeries model docstring to clarify key is primary identifier
- Updated folder field to indicate metadata-only usage
- Updated AnimeSeriesService docstring and get_by_key method
- Updated infrastructure.md with database identifier documentation
- All 996 tests passing

src/server/database/service.py

@@ -43,6 +43,11 @@ class AnimeSeriesService:
     
     Provides methods for creating, reading, updating, and deleting anime series
     with support for both async and sync database sessions.
+    
+    Series Identifier Convention:
+        - Use `get_by_key()` for lookups by provider key (primary identifier)
+        - Use `get_by_id()` for lookups by database primary key (internal)
+        - Never use `folder` for identification - it's metadata only
     """
     
     @staticmethod
@@ -115,12 +120,19 @@ class AnimeSeriesService:
     async def get_by_key(db: AsyncSession, key: str) -> Optional[AnimeSeries]:
         """Get anime series by provider key.
         
+        This is the PRIMARY lookup method for series identification.
+        Use this method instead of get_by_id() when looking up by
+        the provider-assigned unique key.
+        
         Args:
             db: Database session
-            key: Unique provider key
+            key: Unique provider key (e.g., "attack-on-titan")
         
         Returns:
             AnimeSeries instance or None if not found
+        
+        Note:
+            Do NOT use folder for lookups - it's metadata only.
         """
         result = await db.execute(
             select(AnimeSeries).where(AnimeSeries.key == key)