feat(api): add ItemRarity enum to item system

- Add ItemRarity enum with 5 tiers (common, uncommon, rare, epic, legendary)
- Add rarity field to Item dataclass with COMMON default
- Update Item serialization (to_dict/from_dict) for rarity
- Export ItemRarity from models package
- Add 24 comprehensive unit tests for Item and ItemRarity

Part of Phase 4 Week 2: Inventory & Equipment System (Task 2.1)

api/app/models/enums.py

@@ -40,6 +40,16 @@ class ItemType(Enum):
     QUEST_ITEM = "quest_item"   # Story-related, non-tradeable
 
 
+class ItemRarity(Enum):
+    """Item rarity tiers affecting drop rates, value, and visual styling."""
+
+    COMMON = "common"           # White/gray - basic items
+    UNCOMMON = "uncommon"       # Green - slightly better
+    RARE = "rare"               # Blue - noticeably better
+    EPIC = "epic"               # Purple - powerful items
+    LEGENDARY = "legendary"     # Orange/gold - best items
+
+
 class StatType(Enum):
     """Character attribute types."""