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/base_item_loader.py

@@ -109,6 +109,7 @@ class BaseItemLoader:
                 # Set defaults for missing optional fields
                 template_data.setdefault("description", "")
                 template_data.setdefault("base_damage", 0)
+                template_data.setdefault("base_spell_power", 0)
                 template_data.setdefault("base_defense", 0)
                 template_data.setdefault("base_resistance", 0)
                 template_data.setdefault("base_value", 10)