feat(api): implement combat loot integration with hybrid static/procedural system

Add CombatLootService that orchestrates loot generation from combat,
supporting both static item drops (consumables, materials) and procedural
equipment generation (weapons, armor with affixes).

Key changes:
- Extend LootEntry model with LootType enum (STATIC/PROCEDURAL)
- Create StaticItemLoader service for consumables/materials from YAML
- Create CombatLootService with full rarity formula incorporating:
  - Party average level
  - Enemy difficulty tier (EASY: +0%, MEDIUM: +5%, HARD: +15%, BOSS: +30%)
  - Character luck stat
  - Per-entry rarity bonus
- Integrate with CombatService._calculate_rewards() for automatic loot gen
- Add boss guaranteed drops via generate_boss_loot()

New enemy variants (goblin family proof-of-concept):
- goblin_scout (Easy) - static drops only
- goblin_warrior (Medium) - static + procedural weapon drops
- goblin_chieftain (Hard) - static + procedural weapon/armor drops

Static items added:
- consumables.yaml: health/mana potions, elixirs, food
- materials.yaml: trophy items, crafting materials

Tests: 59 new tests across 3 test files (all passing)

api/tests/test_combat_service.py

@@ -623,14 +623,22 @@ class TestRewardsCalculation:
         service = CombatService.__new__(CombatService)
         service.enemy_loader = Mock()
         service.character_service = Mock()
+        service.loot_service = Mock()
 
         # Mock enemy template for rewards
         mock_template = Mock()
         mock_template.experience_reward = 50
         mock_template.get_gold_reward.return_value = 25
-        mock_template.roll_loot.return_value = [{"item_id": "sword", "quantity": 1}]
+        mock_template.difficulty = Mock()
+        mock_template.difficulty.value = "easy"
+        mock_template.is_boss.return_value = False
         service.enemy_loader.load_enemy.return_value = mock_template
 
+        # Mock loot service to return mock items
+        mock_item = Mock()
+        mock_item.to_dict.return_value = {"item_id": "sword", "quantity": 1}
+        service.loot_service.generate_loot_from_enemy.return_value = [mock_item]
+
         mock_session = Mock()
         mock_session.is_solo.return_value = True
         mock_session.solo_character_id = "test_char"