feat(api): add luck (LUK) stat to character system

Add new Luck stat to the character stats system with class-specific values:
- Assassin: 12 (highest - critical specialists)
- Luminary: 11 (divine favor)
- Wildstrider/Lorekeeper: 10 (average)
- Arcanist/Oathkeeper: 9 (modest)
- Vanguard: 8 (default - relies on strength)
- Necromancer: 7 (lowest - dark arts cost)

Changes:
- Add luck field to Stats dataclass with default of 8
- Add LUCK to StatType enum
- Update all 8 class YAML files with luck values
- Display LUK in character panel (play page) and detail page
- Update DATA_MODELS.md documentation

Backward compatible: existing characters without luck default to 8.

api/app/models/enums.py

@@ -49,6 +49,7 @@ class StatType(Enum):
     INTELLIGENCE = "intelligence"   # Magical power
     WISDOM = "wisdom"               # Perception and insight
     CHARISMA = "charisma"           # Social influence
+    LUCK = "luck"                   # Fortune and fate
 
 
 class AbilityType(Enum):