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/data/base_items/weapons.yaml

@@ -146,14 +146,59 @@ weapons:
     name: "Wizard Staff"
     item_type: "weapon"
     description: "A staff attuned to magical energy"
-    base_damage: 8
+    base_damage: 4
+    base_spell_power: 12
     base_value: 45
-    damage_type: "physical"
+    damage_type: "arcane"
     crit_chance: 0.05
     crit_multiplier: 2.0
     required_level: 3
     drop_weight: 0.8
 
+  arcane_staff:
+    template_id: "arcane_staff"
+    name: "Arcane Staff"
+    item_type: "weapon"
+    description: "A powerful staff pulsing with arcane power"
+    base_damage: 6
+    base_spell_power: 18
+    base_value: 90
+    damage_type: "arcane"
+    crit_chance: 0.06
+    crit_multiplier: 2.0
+    required_level: 5
+    drop_weight: 0.6
+    min_rarity: "uncommon"
+
+  # ==================== WANDS ====================
+  wand:
+    template_id: "wand"
+    name: "Wand"
+    item_type: "weapon"
+    description: "A simple magical focus"
+    base_damage: 2
+    base_spell_power: 8
+    base_value: 30
+    damage_type: "arcane"
+    crit_chance: 0.06
+    crit_multiplier: 2.0
+    required_level: 1
+    drop_weight: 1.0
+
+  crystal_wand:
+    template_id: "crystal_wand"
+    name: "Crystal Wand"
+    item_type: "weapon"
+    description: "A wand topped with a magical crystal"
+    base_damage: 3
+    base_spell_power: 14
+    base_value: 60
+    damage_type: "arcane"
+    crit_chance: 0.07
+    crit_multiplier: 2.2
+    required_level: 4
+    drop_weight: 0.8
+
   # ==================== RANGED ====================
   shortbow:
     template_id: "shortbow"