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/stats.py

@@ -21,6 +21,7 @@ class Stats:
         intelligence: Magical power, affects spell damage and MP
         wisdom: Perception and insight, affects magical resistance
         charisma: Social influence, affects NPC interactions
+        luck: Fortune and fate, affects critical hits, loot, and random outcomes
 
     Computed Properties:
         hit_points: Maximum HP = 10 + (constitution × 2)
@@ -35,6 +36,7 @@ class Stats:
     intelligence: int = 10
     wisdom: int = 10
     charisma: int = 10
+    luck: int = 8
 
     @property
     def hit_points(self) -> int:
@@ -111,6 +113,7 @@ class Stats:
             intelligence=data.get("intelligence", 10),
             wisdom=data.get("wisdom", 10),
             charisma=data.get("charisma", 10),
+            luck=data.get("luck", 8),
         )
 
     def copy(self) -> 'Stats':
@@ -127,6 +130,7 @@ class Stats:
             intelligence=self.intelligence,
             wisdom=self.wisdom,
             charisma=self.charisma,
+            luck=self.luck,
         )
 
     def __repr__(self) -> str:
@@ -134,7 +138,7 @@ class Stats:
         return (
             f"Stats(STR={self.strength}, DEX={self.dexterity}, "
             f"CON={self.constitution}, INT={self.intelligence}, "
-            f"WIS={self.wisdom}, CHA={self.charisma}, "
+            f"WIS={self.wisdom}, CHA={self.charisma}, LUK={self.luck}, "
             f"HP={self.hit_points}, MP={self.mana_points}, "
             f"DEF={self.defense}, RES={self.resistance})"
         )