feat: Task 1.1 - Enforce key as primary identifier in Serie class

- Add validation in Serie.__init__ to prevent empty/whitespace keys
- Add validation in Serie.key setter to prevent empty values
- Automatically strip whitespace from key values
- Add comprehensive docstrings explaining key as unique identifier
- Document folder property as metadata only (not for lookups)
- Create comprehensive test suite with 16 tests in test_serie_class.py
- All 56 Serie-related tests pass successfully
- Update instructions.md to mark Task 1.1 as completed

This is the first task in the Series Identifier Standardization effort
to establish 'key' as the single source of truth for series identification
throughout the codebase.

src/core/entities/series.py

@@ -1,23 +1,65 @@
 import json
 
+
 class Serie:
+    """
+    Represents an anime series with metadata and episode information.
+    
+    The `key` property is the unique identifier for the series (provider-assigned, URL-safe).
+    The `folder` property is the filesystem folder name (metadata only, not used for lookups).
+    
+    Args:
+        key: Unique series identifier from provider (e.g., "attack-on-titan"). Cannot be empty.
+        name: Display name of the series
+        site: Provider site URL
+        folder: Filesystem folder name (metadata only, e.g., "Attack on Titan (2013)")
+        episodeDict: Dictionary mapping season numbers to lists of episode numbers
+        
+    Raises:
+        ValueError: If key is None or empty string
+    """
+    
     def __init__(self, key: str, name: str, site: str, folder: str, episodeDict: dict[int, list[int]]):
-        self._key = key
+        if not key or not key.strip():
+            raise ValueError("Serie key cannot be None or empty")
+        
+        self._key = key.strip()
         self._name = name
         self._site = site
         self._folder = folder
         self._episodeDict = episodeDict
+        
     def __str__(self):
         """String representation of Serie object"""
         return f"Serie(key='{self.key}', name='{self.name}', site='{self.site}', folder='{self.folder}', episodeDict={self.episodeDict})"
 
     @property
     def key(self) -> str:
+        """
+        Unique series identifier (primary identifier for all lookups).
+        
+        This is the provider-assigned, URL-safe identifier used throughout the application
+        for series identification, lookups, and operations.
+        
+        Returns:
+            str: The unique series key
+        """
         return self._key
 
     @key.setter
     def key(self, value: str):
-        self._key = value
+        """
+        Set the unique series identifier.
+        
+        Args:
+            value: New key value
+            
+        Raises:
+            ValueError: If value is None or empty string
+        """
+        if not value or not value.strip():
+            raise ValueError("Serie key cannot be None or empty")
+        self._key = value.strip()
 
     @property
     def name(self) -> str:
@@ -37,10 +79,26 @@ class Serie:
 
     @property
     def folder(self) -> str:
+        """
+        Filesystem folder name (metadata only, not used for lookups).
+        
+        This property contains the local directory name where the series
+        files are stored. It should NOT be used as an identifier for
+        series lookups - use `key` instead.
+        
+        Returns:
+            str: The filesystem folder name
+        """
         return self._folder
 
     @folder.setter
     def folder(self, value: str):
+        """
+        Set the filesystem folder name.
+        
+        Args:
+            value: Folder name for the series
+        """
         self._folder = value
 
     @property