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.

public_web/templates/character/detail.html

@@ -81,6 +81,10 @@
                     <span class="stat-name">CHA</span>
                     <span class="stat-value">{{ character.base_stats.charisma }}</span>
                 </div>
+                <div class="stat-item">
+                    <span class="stat-name">LUK</span>
+                    <span class="stat-value">{{ character.base_stats.luck }}</span>
+                </div>
             </div>
 
             <!-- Derived Stats -->