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/services/combat_service.py

@@ -571,17 +571,26 @@ class CombatService:
                 message="Invalid or dead target"
             )
 
-        # Get attacker's weapon damage (or base damage for enemies)
-        weapon_damage = self._get_weapon_damage(attacker)
-        crit_chance = self._get_crit_chance(attacker)
-
-        # Calculate damage using DamageCalculator
-        damage_result = DamageCalculator.calculate_physical_damage(
-            attacker_stats=attacker.stats,
-            defender_stats=target.stats,
-            weapon_damage=weapon_damage,
-            weapon_crit_chance=crit_chance,
-        )
+        # Check if this is an elemental weapon attack
+        if attacker.elemental_ratio > 0.0 and attacker.elemental_damage_type:
+            # Elemental weapon: split damage between physical and elemental
+            damage_result = DamageCalculator.calculate_elemental_weapon_damage(
+                attacker_stats=attacker.stats,
+                defender_stats=target.stats,
+                weapon_crit_chance=attacker.weapon_crit_chance,
+                weapon_crit_multiplier=attacker.weapon_crit_multiplier,
+                physical_ratio=attacker.physical_ratio,
+                elemental_ratio=attacker.elemental_ratio,
+                elemental_type=attacker.elemental_damage_type,
+            )
+        else:
+            # Normal physical attack
+            damage_result = DamageCalculator.calculate_physical_damage(
+                attacker_stats=attacker.stats,
+                defender_stats=target.stats,
+                weapon_crit_chance=attacker.weapon_crit_chance,
+                weapon_crit_multiplier=attacker.weapon_crit_multiplier,
+            )
 
         # Add target_id to result for tracking
         damage_result.target_id = target.combatant_id
@@ -970,6 +979,25 @@ class CombatService:
         abilities = ["basic_attack"]  # All characters have basic attack
         abilities.extend(character.unlocked_skills)
 
+        # Extract weapon properties from equipped weapon
+        weapon = character.equipped.get("weapon")
+        weapon_crit_chance = 0.05
+        weapon_crit_multiplier = 2.0
+        weapon_damage_type = DamageType.PHYSICAL
+        elemental_damage_type = None
+        physical_ratio = 1.0
+        elemental_ratio = 0.0
+
+        if weapon and weapon.is_weapon():
+            weapon_crit_chance = weapon.crit_chance
+            weapon_crit_multiplier = weapon.crit_multiplier
+            weapon_damage_type = weapon.damage_type or DamageType.PHYSICAL
+
+            if weapon.is_elemental_weapon():
+                elemental_damage_type = weapon.elemental_damage_type
+                physical_ratio = weapon.physical_ratio
+                elemental_ratio = weapon.elemental_ratio
+
         return Combatant(
             combatant_id=character.character_id,
             name=character.name,
@@ -980,6 +1008,12 @@ class CombatService:
             max_mp=effective_stats.mana_points,
             stats=effective_stats,
             abilities=abilities,
+            weapon_crit_chance=weapon_crit_chance,
+            weapon_crit_multiplier=weapon_crit_multiplier,
+            weapon_damage_type=weapon_damage_type,
+            elemental_damage_type=elemental_damage_type,
+            physical_ratio=physical_ratio,
+            elemental_ratio=elemental_ratio,
         )
 
     def _create_combatant_from_enemy(
@@ -996,7 +1030,9 @@ class CombatService:
         if instance_index > 0:
             name = f"{template.name} #{instance_index + 1}"
 
-        stats = template.base_stats
+        # Copy stats and populate damage_bonus with base_damage
+        stats = template.base_stats.copy()
+        stats.damage_bonus = template.base_damage
 
         return Combatant(
             combatant_id=combatant_id,
@@ -1008,23 +1044,15 @@ class CombatService:
             max_mp=stats.mana_points,
             stats=stats,
             abilities=template.abilities.copy(),
+            weapon_crit_chance=template.crit_chance,
+            weapon_crit_multiplier=2.0,
+            weapon_damage_type=DamageType.PHYSICAL,
         )
 
-    def _get_weapon_damage(self, combatant: Combatant) -> int:
-        """Get weapon damage for a combatant."""
-        # For enemies, use base_damage from template
-        if not combatant.is_player:
-            # Base damage stored in combatant data or default
-            return 8  # Default enemy damage
-
-        # For players, would check equipped weapon
-        # TODO: Check character's equipped weapon
-        return 5  # Default unarmed damage
-
     def _get_crit_chance(self, combatant: Combatant) -> float:
         """Get critical hit chance for a combatant."""
-        # Base 5% + LUK bonus
-        return 0.05 + combatant.stats.crit_bonus
+        # Weapon crit chance + LUK bonus
+        return combatant.weapon_crit_chance + combatant.stats.crit_bonus
 
     def _get_default_target(
         self,