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/static_items/consumables.yaml

@@ -0,0 +1,161 @@
+# Consumable items that drop from enemies or are purchased from vendors
+# These items have effects_on_use that trigger when consumed
+
+items:
+  # ==========================================================================
+  # Health Potions
+  # ==========================================================================
+
+  health_potion_small:
+    name: "Small Health Potion"
+    item_type: consumable
+    rarity: common
+    description: "A small vial of red liquid that restores a modest amount of health."
+    value: 25
+    is_tradeable: true
+    effects_on_use:
+      - effect_id: heal_small
+        name: "Minor Healing"
+        effect_type: hot
+        power: 30
+        duration: 1
+        stacks: 1
+
+  health_potion_medium:
+    name: "Health Potion"
+    item_type: consumable
+    rarity: uncommon
+    description: "A standard healing potion used by adventurers across the realm."
+    value: 75
+    is_tradeable: true
+    effects_on_use:
+      - effect_id: heal_medium
+        name: "Healing"
+        effect_type: hot
+        power: 75
+        duration: 1
+        stacks: 1
+
+  health_potion_large:
+    name: "Large Health Potion"
+    item_type: consumable
+    rarity: rare
+    description: "A potent healing draught that restores significant health."
+    value: 150
+    is_tradeable: true
+    effects_on_use:
+      - effect_id: heal_large
+        name: "Major Healing"
+        effect_type: hot
+        power: 150
+        duration: 1
+        stacks: 1
+
+  # ==========================================================================
+  # Mana Potions
+  # ==========================================================================
+
+  mana_potion_small:
+    name: "Small Mana Potion"
+    item_type: consumable
+    rarity: common
+    description: "A small vial of blue liquid that restores mana."
+    value: 25
+    is_tradeable: true
+    # Note: MP restoration would need custom effect type or game logic
+
+  mana_potion_medium:
+    name: "Mana Potion"
+    item_type: consumable
+    rarity: uncommon
+    description: "A standard mana potion favored by spellcasters."
+    value: 75
+    is_tradeable: true
+
+  # ==========================================================================
+  # Status Effect Cures
+  # ==========================================================================
+
+  antidote:
+    name: "Antidote"
+    item_type: consumable
+    rarity: common
+    description: "A bitter herbal remedy that cures poison effects."
+    value: 30
+    is_tradeable: true
+
+  smelling_salts:
+    name: "Smelling Salts"
+    item_type: consumable
+    rarity: uncommon
+    description: "Pungent salts that can revive unconscious allies or cure stun."
+    value: 40
+    is_tradeable: true
+
+  # ==========================================================================
+  # Combat Buffs
+  # ==========================================================================
+
+  elixir_of_strength:
+    name: "Elixir of Strength"
+    item_type: consumable
+    rarity: rare
+    description: "A powerful elixir that temporarily increases strength."
+    value: 100
+    is_tradeable: true
+    effects_on_use:
+      - effect_id: str_buff
+        name: "Strength Boost"
+        effect_type: buff
+        power: 5
+        duration: 5
+        stacks: 1
+
+  elixir_of_agility:
+    name: "Elixir of Agility"
+    item_type: consumable
+    rarity: rare
+    description: "A shimmering elixir that enhances reflexes and speed."
+    value: 100
+    is_tradeable: true
+    effects_on_use:
+      - effect_id: dex_buff
+        name: "Agility Boost"
+        effect_type: buff
+        power: 5
+        duration: 5
+        stacks: 1
+
+  # ==========================================================================
+  # Food Items (simple healing, no combat use)
+  # ==========================================================================
+
+  ration:
+    name: "Trail Ration"
+    item_type: consumable
+    rarity: common
+    description: "Dried meat, hardtack, and nuts. Sustains an adventurer on long journeys."
+    value: 5
+    is_tradeable: true
+    effects_on_use:
+      - effect_id: ration_heal
+        name: "Nourishment"
+        effect_type: hot
+        power: 10
+        duration: 1
+        stacks: 1
+
+  cooked_meat:
+    name: "Cooked Meat"
+    item_type: consumable
+    rarity: common
+    description: "Freshly cooked meat that restores health."
+    value: 15
+    is_tradeable: true
+    effects_on_use:
+      - effect_id: meat_heal
+        name: "Hearty Meal"
+        effect_type: hot
+        power: 20
+        duration: 1
+        stacks: 1