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/app/data/enemies/goblin_chieftain.yaml

@@ -0,0 +1,85 @@
+# Goblin Chieftain - Hard variant, elite tribe leader
+# A cunning and powerful goblin leader, adorned with trophies.
+# Commands respect through fear and violence, drops quality loot.
+
+enemy_id: goblin_chieftain
+name: Goblin Chieftain
+description: >
+  A large, scarred goblin wearing a crown of teeth and bones.
+  The chieftain has clawed its way to leadership through countless
+  battles and betrayals. It wields a well-maintained weapon stolen
+  from a fallen adventurer and commands its tribe with an iron fist.
+
+base_stats:
+  strength: 16
+  dexterity: 12
+  constitution: 14
+  intelligence: 10
+  wisdom: 10
+  charisma: 12
+  luck: 12
+
+abilities:
+  - basic_attack
+  - shield_bash
+  - intimidating_shout
+
+loot_table:
+  # Static drops - guaranteed materials
+  - loot_type: static
+    item_id: goblin_ear
+    drop_chance: 1.0
+    quantity_min: 2
+    quantity_max: 3
+  - loot_type: static
+    item_id: goblin_chieftain_token
+    drop_chance: 0.80
+    quantity_min: 1
+    quantity_max: 1
+  - loot_type: static
+    item_id: goblin_war_paint
+    drop_chance: 0.50
+    quantity_min: 1
+    quantity_max: 2
+
+  # Consumable drops
+  - loot_type: static
+    item_id: health_potion_medium
+    drop_chance: 0.40
+    quantity_min: 1
+    quantity_max: 1
+  - loot_type: static
+    item_id: elixir_of_strength
+    drop_chance: 0.10
+    quantity_min: 1
+    quantity_max: 1
+
+  # Procedural equipment drops - higher chance and rarity bonus
+  - loot_type: procedural
+    item_type: weapon
+    drop_chance: 0.25
+    rarity_bonus: 0.10
+    quantity_min: 1
+    quantity_max: 1
+  - loot_type: procedural
+    item_type: armor
+    drop_chance: 0.15
+    rarity_bonus: 0.05
+    quantity_min: 1
+    quantity_max: 1
+
+experience_reward: 75
+gold_reward_min: 20
+gold_reward_max: 50
+difficulty: hard
+
+tags:
+  - humanoid
+  - goblinoid
+  - leader
+  - elite
+  - armed
+
+base_damage: 14
+crit_chance: 0.15
+flee_chance: 0.25