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/__init__.py

@@ -9,6 +9,7 @@ from app.models.enums import (
     EffectType,
     DamageType,
     ItemType,
+    ItemRarity,
     StatType,
     AbilityType,
     CombatStatus,
@@ -53,6 +54,7 @@ __all__ = [
     "EffectType",
     "DamageType",
     "ItemType",
+    "ItemRarity",
     "StatType",
     "AbilityType",
     "CombatStatus",