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/arcanist.yaml

@@ -8,7 +8,7 @@ description: >
   excel in devastating spell damage, capable of incinerating groups of foes or freezing
   enemies in place. Choose your element: embrace the flames or command the frost.
 
-# Base stats (total: 65)
+# Base stats (total: 65 + luck)
 base_stats:
   strength: 8       # Low physical power
   dexterity: 10     # Average agility
@@ -16,6 +16,7 @@ base_stats:
   intelligence: 15  # Exceptional magical power
   wisdom: 12        # Above average perception
   charisma: 11      # Above average social
+  luck: 9           # Slight chaos magic boost
 
 starting_equipment:
   - worn_staff

api/app/data/classes/assassin.yaml

@@ -8,7 +8,7 @@ description: >
   capable of becoming an elusive phantom or a master of critical strikes. Choose your path: embrace
   the shadows or perfect the killing blow.
 
-# Base stats (total: 65)
+# Base stats (total: 65 + luck)
 base_stats:
   strength: 11      # Above average physical power
   dexterity: 15     # Exceptional agility
@@ -16,6 +16,7 @@ base_stats:
   intelligence: 9   # Below average magic
   wisdom: 10        # Average perception
   charisma: 10      # Average social
+  luck: 12          # High luck for crits and precision
 
 starting_equipment:
   - rusty_dagger

api/app/data/classes/lorekeeper.yaml

@@ -8,7 +8,7 @@ description: >
   excel in supporting allies and controlling enemies through clever magic and mental manipulation.
   Choose your art: weave arcane power or bend reality itself.
 
-# Base stats (total: 67)
+# Base stats (total: 67 + luck)
 base_stats:
   strength: 8       # Low physical power
   dexterity: 11     # Above average agility
@@ -16,6 +16,7 @@ base_stats:
   intelligence: 13  # Above average magical power
   wisdom: 11        # Above average perception
   charisma: 14      # High social/performance
+  luck: 10          # Knowledge is its own luck
 
 starting_equipment:
   - tome

api/app/data/classes/luminary.yaml

@@ -8,7 +8,7 @@ description: >
   capable of becoming a guardian angel for their allies or a righteous crusader smiting evil.
   Choose your calling: protect the innocent or judge the wicked.
 
-# Base stats (total: 68)
+# Base stats (total: 68 + luck)
 base_stats:
   strength: 9       # Below average physical power
   dexterity: 9      # Below average agility
@@ -16,6 +16,7 @@ base_stats:
   intelligence: 12  # Above average magical power
   wisdom: 14        # High perception/divine power
   charisma: 13      # Above average social
+  luck: 11          # Divine favor grants fortune
 
 starting_equipment:
   - rusty_mace

api/app/data/classes/necromancer.yaml

@@ -8,7 +8,7 @@ description: >
   excel in draining enemies over time or overwhelming foes with undead minions.
   Choose your dark art: curse your enemies or raise an army of the dead.
 
-# Base stats (total: 65)
+# Base stats (total: 65 + luck)
 base_stats:
   strength: 8       # Low physical power
   dexterity: 10     # Average agility
@@ -16,6 +16,7 @@ base_stats:
   intelligence: 14  # High magical power
   wisdom: 11        # Above average perception
   charisma: 12      # Above average social (commands undead)
+  luck: 7           # Dark arts come with a cost
 
 starting_equipment:
   - bone_wand

api/app/data/classes/oathkeeper.yaml

@@ -8,7 +8,7 @@ description: >
   capable of becoming an unyielding shield for their allies or a beacon of healing light.
   Choose your oath: defend the weak or redeem the fallen.
 
-# Base stats (total: 67)
+# Base stats (total: 67 + luck)
 base_stats:
   strength: 12      # Above average physical power
   dexterity: 9      # Below average agility
@@ -16,6 +16,7 @@ base_stats:
   intelligence: 10  # Average magic
   wisdom: 12        # Above average perception
   charisma: 11      # Above average social
+  luck: 9           # Honorable, modest fortune
 
 starting_equipment:
   - rusty_sword

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:

api/app/data/classes/wildstrider.yaml

@@ -8,7 +8,7 @@ description: >
   can become elite marksmen with unmatched accuracy or beast masters commanding powerful
   animal companions. Choose your path: perfect your aim or unleash the wild.
 
-# Base stats (total: 66)
+# Base stats (total: 66 + luck)
 base_stats:
   strength: 10      # Average physical power
   dexterity: 14     # High agility
@@ -16,6 +16,7 @@ base_stats:
   intelligence: 9   # Below average magic
   wisdom: 13        # Above average perception
   charisma: 9       # Below average social
+  luck: 10          # Average luck, self-reliant
 
 starting_equipment:
   - rusty_bow