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_scout.yaml

@@ -0,0 +1,56 @@
+# Goblin Scout - Easy variant, agile but fragile
+# A fast, cowardly goblin that serves as a lookout for its tribe.
+# Quick to flee, drops minor loot and the occasional small potion.
+
+enemy_id: goblin_scout
+name: Goblin Scout
+description: >
+  A small, wiry goblin with oversized ears and beady yellow eyes.
+  Goblin scouts are the first line of awareness for their tribes,
+  often found lurking in shadows or perched in trees. They prefer
+  to run rather than fight, but will attack if cornered.
+
+base_stats:
+  strength: 6
+  dexterity: 14
+  constitution: 5
+  intelligence: 6
+  wisdom: 10
+  charisma: 4
+  luck: 10
+
+abilities:
+  - basic_attack
+
+loot_table:
+  # Static drops - materials and consumables
+  - loot_type: static
+    item_id: goblin_ear
+    drop_chance: 0.60
+    quantity_min: 1
+    quantity_max: 1
+  - loot_type: static
+    item_id: goblin_trinket
+    drop_chance: 0.20
+    quantity_min: 1
+    quantity_max: 1
+  - loot_type: static
+    item_id: health_potion_small
+    drop_chance: 0.08
+    quantity_min: 1
+    quantity_max: 1
+
+experience_reward: 10
+gold_reward_min: 1
+gold_reward_max: 4
+difficulty: easy
+
+tags:
+  - humanoid
+  - goblinoid
+  - small
+  - scout
+
+base_damage: 3
+crit_chance: 0.08
+flee_chance: 0.70