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/data/classes/vanguard.yaml

@@ -8,7 +8,7 @@ description: >
   capable of becoming an unbreakable shield for their allies or a relentless damage dealer.
   Choose your path: become a stalwart defender or a devastating weapon master.
 
-# Base stats (total: 65, average: 10.83)
+# Base stats (total: 65 + luck)
 base_stats:
   strength: 14      # High physical power
   dexterity: 10     # Average agility
@@ -16,6 +16,7 @@ base_stats:
   intelligence: 8   # Low magic
   wisdom: 10        # Average perception
   charisma: 9       # Below average social
+  luck: 8           # Low luck, relies on strength
 
 # Starting equipment (minimal)
 starting_equipment: