feat(api): integrate equipment stats into combat damage system

Equipment-Combat Integration:
- Update Stats damage formula from STR//2 to int(STR*0.75) for better scaling
- Add spell_power system for magical weapons (staves, wands)
- Add spell_power_bonus field to Stats model with spell_power property
- Add spell_power field to Item model with is_magical_weapon() method
- Update Character.get_effective_stats() to populate spell_power_bonus

Combatant Model Updates:
- Add weapon property fields (crit_chance, crit_multiplier, damage_type)
- Add elemental weapon support (elemental_damage_type, physical_ratio, elemental_ratio)
- Update serialization to handle new weapon properties

DamageCalculator Refactoring:
- Remove weapon_damage parameter from calculate_physical_damage()
- Use attacker_stats.damage directly (includes weapon bonus)
- Use attacker_stats.spell_power for magical damage calculations

Combat Service Updates:
- Extract weapon properties in _create_combatant_from_character()
- Use stats.damage_bonus for enemy combatants from templates
- Remove hardcoded _get_weapon_damage() method
- Handle elemental weapons with split damage in _execute_attack()

Item Generation Updates:
- Add base_spell_power to BaseItemTemplate dataclass
- Add ARCANE damage type to DamageType enum
- Add magical weapon templates (wizard_staff, arcane_staff, wand, crystal_wand)

Test Updates:
- Update test_stats.py for new damage formula (0.75 scaling)
- Update test_character.py for equipment bonus calculations
- Update test_damage_calculator.py for new API signatures
- Update test_combat_service.py mock fixture for equipped attribute

Tests: 174 passing

api/app/models/items.py

@@ -33,7 +33,8 @@ class Item:
         effects_on_use: Effects applied when consumed (consumables only)
 
         Weapon-specific attributes:
-        damage: Base weapon damage
+        damage: Base weapon damage (physical/melee/ranged)
+        spell_power: Spell power for staves/wands (boosts spell damage)
         damage_type: Type of damage (physical, fire, etc.)
         crit_chance: Probability of critical hit (0.0 to 1.0)
         crit_multiplier: Damage multiplier on critical hit
@@ -62,7 +63,8 @@ class Item:
     effects_on_use: List[Effect] = field(default_factory=list)
 
     # Weapon-specific
-    damage: int = 0
+    damage: int = 0  # Physical damage for melee/ranged weapons
+    spell_power: int = 0  # Spell power for staves/wands (boosts spell damage)
     damage_type: Optional[DamageType] = None
     crit_chance: float = 0.05  # 5% default critical hit chance
     crit_multiplier: float = 2.0  # 2x damage on critical hit
@@ -136,6 +138,18 @@ class Item:
             self.elemental_damage_type is not None
         )
 
+    def is_magical_weapon(self) -> bool:
+        """
+        Check if this weapon is a spell-casting weapon (staff, wand, tome).
+
+        Magical weapons provide spell_power which boosts spell damage,
+        rather than physical damage for melee/ranged attacks.
+
+        Returns:
+            True if weapon has spell_power (staves, wands, etc.)
+        """
+        return self.is_weapon() and self.spell_power > 0
+
     def can_equip(self, character_level: int, character_class: Optional[str] = None) -> bool:
         """
         Check if a character can equip this item.