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/tests/test_character.py

@@ -452,3 +452,183 @@ def test_character_round_trip_serialization(basic_character):
     assert restored.unlocked_skills == basic_character.unlocked_skills
     assert "weapon" in restored.equipped
     assert restored.equipped["weapon"].item_id == "sword"
+
+
+# =============================================================================
+# Equipment Combat Bonuses (Task 2.5)
+# =============================================================================
+
+def test_get_effective_stats_weapon_damage_bonus(basic_character):
+    """Test that weapon damage is added to effective stats damage_bonus."""
+    # Create weapon with damage
+    weapon = Item(
+        item_id="iron_sword",
+        name="Iron Sword",
+        item_type=ItemType.WEAPON,
+        description="A sturdy iron sword",
+        damage=15,  # 15 damage
+    )
+
+    basic_character.equipped["weapon"] = weapon
+    effective = basic_character.get_effective_stats()
+
+    # Base strength is 12, so base damage = int(12 * 0.75) = 9
+    # Weapon damage = 15
+    # Total damage property = 9 + 15 = 24
+    assert effective.damage_bonus == 15
+    assert effective.damage == 24  # int(12 * 0.75) + 15
+
+
+def test_get_effective_stats_armor_defense_bonus(basic_character):
+    """Test that armor defense is added to effective stats defense_bonus."""
+    # Create armor with defense
+    armor = Item(
+        item_id="iron_chestplate",
+        name="Iron Chestplate",
+        item_type=ItemType.ARMOR,
+        description="A sturdy iron chestplate",
+        defense=10,
+        resistance=0,
+    )
+
+    basic_character.equipped["chest"] = armor
+    effective = basic_character.get_effective_stats()
+
+    # Base constitution is 14, so base defense = 14 // 2 = 7
+    # Armor defense = 10
+    # Total defense property = 7 + 10 = 17
+    assert effective.defense_bonus == 10
+    assert effective.defense == 17  # (14 // 2) + 10
+
+
+def test_get_effective_stats_armor_resistance_bonus(basic_character):
+    """Test that armor resistance is added to effective stats resistance_bonus."""
+    # Create armor with resistance
+    robe = Item(
+        item_id="magic_robe",
+        name="Magic Robe",
+        item_type=ItemType.ARMOR,
+        description="An enchanted robe",
+        defense=2,
+        resistance=8,
+    )
+
+    basic_character.equipped["chest"] = robe
+    effective = basic_character.get_effective_stats()
+
+    # Base wisdom is 10, so base resistance = 10 // 2 = 5
+    # Armor resistance = 8
+    # Total resistance property = 5 + 8 = 13
+    assert effective.resistance_bonus == 8
+    assert effective.resistance == 13  # (10 // 2) + 8
+
+
+def test_get_effective_stats_multiple_armor_pieces(basic_character):
+    """Test that multiple armor pieces stack their bonuses."""
+    # Create multiple armor pieces
+    helmet = Item(
+        item_id="iron_helmet",
+        name="Iron Helmet",
+        item_type=ItemType.ARMOR,
+        description="Protects your head",
+        defense=5,
+        resistance=2,
+    )
+
+    chestplate = Item(
+        item_id="iron_chestplate",
+        name="Iron Chestplate",
+        item_type=ItemType.ARMOR,
+        description="Protects your torso",
+        defense=10,
+        resistance=3,
+    )
+
+    boots = Item(
+        item_id="iron_boots",
+        name="Iron Boots",
+        item_type=ItemType.ARMOR,
+        description="Protects your feet",
+        defense=3,
+        resistance=1,
+    )
+
+    basic_character.equipped["helmet"] = helmet
+    basic_character.equipped["chest"] = chestplate
+    basic_character.equipped["boots"] = boots
+
+    effective = basic_character.get_effective_stats()
+
+    # Total defense bonus = 5 + 10 + 3 = 18
+    # Total resistance bonus = 2 + 3 + 1 = 6
+    assert effective.defense_bonus == 18
+    assert effective.resistance_bonus == 6
+
+    # Base constitution is 14: base defense = 7
+    # Base wisdom is 10: base resistance = 5
+    assert effective.defense == 25  # 7 + 18
+    assert effective.resistance == 11  # 5 + 6
+
+
+def test_get_effective_stats_weapon_and_armor_combined(basic_character):
+    """Test that weapon damage and armor defense/resistance work together."""
+    # Create weapon
+    weapon = Item(
+        item_id="flaming_sword",
+        name="Flaming Sword",
+        item_type=ItemType.WEAPON,
+        description="A sword wreathed in flame",
+        damage=18,
+        stat_bonuses={"strength": 3},  # Also has stat bonus
+    )
+
+    # Create armor
+    armor = Item(
+        item_id="dragon_armor",
+        name="Dragon Armor",
+        item_type=ItemType.ARMOR,
+        description="Forged from dragon scales",
+        defense=15,
+        resistance=10,
+        stat_bonuses={"constitution": 2},  # Also has stat bonus
+    )
+
+    basic_character.equipped["weapon"] = weapon
+    basic_character.equipped["chest"] = armor
+
+    effective = basic_character.get_effective_stats()
+
+    # Weapon: damage=18, +3 STR
+    # Armor: defense=15, resistance=10, +2 CON
+    # Base STR=12 -> 12+3=15, damage = int(15 * 0.75) + 18 = 11 + 18 = 29
+    assert effective.strength == 15
+    assert effective.damage_bonus == 18
+    assert effective.damage == 29
+
+    # Base CON=14 -> 14+2=16, defense = (16//2) + 15 = 8 + 15 = 23
+    assert effective.constitution == 16
+    assert effective.defense_bonus == 15
+    assert effective.defense == 23
+
+    # Base WIS=10, resistance = (10//2) + 10 = 5 + 10 = 15
+    assert effective.resistance_bonus == 10
+    assert effective.resistance == 15
+
+
+def test_get_effective_stats_no_equipment_bonuses(basic_character):
+    """Test that bonus fields are zero when no equipment is equipped."""
+    effective = basic_character.get_effective_stats()
+
+    assert effective.damage_bonus == 0
+    assert effective.defense_bonus == 0
+    assert effective.resistance_bonus == 0
+
+    # Damage/defense/resistance should just be base stat derived values
+    # Base STR=12, damage = int(12 * 0.75) = 9
+    assert effective.damage == 9
+
+    # Base CON=14, defense = 14 // 2 = 7
+    assert effective.defense == 7
+
+    # Base WIS=10, resistance = 10 // 2 = 5
+    assert effective.resistance == 5