feat(api): implement Diablo-style item affix system

Add procedural item generation with affix naming system:
- Items with RARE/EPIC/LEGENDARY rarity get dynamic names
- Prefixes (e.g., "Flaming") add elemental damage, material bonuses
- Suffixes (e.g., "of Strength") add stat bonuses
- Affix count scales with rarity: RARE=1, EPIC=2, LEGENDARY=3

New files:
- models/affixes.py: Affix and BaseItemTemplate dataclasses
- services/affix_loader.py: YAML-based affix pool loading
- services/base_item_loader.py: Base item template loading
- services/item_generator.py: Main procedural generation service
- data/affixes/prefixes.yaml: 14 prefix definitions
- data/affixes/suffixes.yaml: 15 suffix definitions
- data/base_items/weapons.yaml: 12 weapon templates
- data/base_items/armor.yaml: 12 armor templates
- tests/test_item_generator.py: 34 comprehensive tests

Modified:
- enums.py: Added AffixType and AffixTier enums
- items.py: Added affix tracking fields (applied_affixes, generated_name)

Example output: "Frozen Dagger of the Bear" (EPIC with ice damage + STR/CON)

api/app/data/base_items/armor.yaml

@@ -0,0 +1,152 @@
+# Base Armor Templates for Procedural Generation
+#
+# These templates define the foundation that affixes attach to.
+# Example: "Leather Vest" + "Sturdy" prefix = "Sturdy Leather Vest"
+#
+# Armor categories:
+# - Cloth: Low defense, high resistance (mages)
+# - Leather: Balanced defense/resistance (rogues)
+# - Chain: Medium defense, low resistance (versatile)
+# - Plate: High defense, low resistance (warriors)
+
+armor:
+  # ==================== CLOTH (MAGE ARMOR) ====================
+  cloth_robe:
+    template_id: "cloth_robe"
+    name: "Cloth Robe"
+    item_type: "armor"
+    description: "Simple cloth robes favored by spellcasters"
+    base_defense: 2
+    base_resistance: 5
+    base_value: 15
+    required_level: 1
+    drop_weight: 1.3
+
+  silk_robe:
+    template_id: "silk_robe"
+    name: "Silk Robe"
+    item_type: "armor"
+    description: "Fine silk robes that channel magical energy"
+    base_defense: 3
+    base_resistance: 8
+    base_value: 40
+    required_level: 3
+    drop_weight: 0.9
+
+  arcane_vestments:
+    template_id: "arcane_vestments"
+    name: "Arcane Vestments"
+    item_type: "armor"
+    description: "Robes woven with magical threads"
+    base_defense: 5
+    base_resistance: 12
+    base_value: 80
+    required_level: 5
+    drop_weight: 0.6
+    min_rarity: "uncommon"
+
+  # ==================== LEATHER (ROGUE ARMOR) ====================
+  leather_vest:
+    template_id: "leather_vest"
+    name: "Leather Vest"
+    item_type: "armor"
+    description: "Basic leather protection for agile fighters"
+    base_defense: 5
+    base_resistance: 2
+    base_value: 20
+    required_level: 1
+    drop_weight: 1.3
+
+  studded_leather:
+    template_id: "studded_leather"
+    name: "Studded Leather"
+    item_type: "armor"
+    description: "Leather armor reinforced with metal studs"
+    base_defense: 8
+    base_resistance: 3
+    base_value: 45
+    required_level: 3
+    drop_weight: 1.0
+
+  hardened_leather:
+    template_id: "hardened_leather"
+    name: "Hardened Leather"
+    item_type: "armor"
+    description: "Boiled and hardened leather for superior protection"
+    base_defense: 12
+    base_resistance: 5
+    base_value: 75
+    required_level: 5
+    drop_weight: 0.7
+    min_rarity: "uncommon"
+
+  # ==================== CHAIN (VERSATILE) ====================
+  chain_shirt:
+    template_id: "chain_shirt"
+    name: "Chain Shirt"
+    item_type: "armor"
+    description: "A shirt of interlocking metal rings"
+    base_defense: 7
+    base_resistance: 2
+    base_value: 35
+    required_level: 2
+    drop_weight: 1.0
+
+  chainmail:
+    template_id: "chainmail"
+    name: "Chainmail"
+    item_type: "armor"
+    description: "Full chainmail armor covering torso and arms"
+    base_defense: 10
+    base_resistance: 3
+    base_value: 50
+    required_level: 3
+    drop_weight: 1.0
+
+  heavy_chainmail:
+    template_id: "heavy_chainmail"
+    name: "Heavy Chainmail"
+    item_type: "armor"
+    description: "Thick chainmail with reinforced rings"
+    base_defense: 14
+    base_resistance: 4
+    base_value: 85
+    required_level: 5
+    drop_weight: 0.7
+    min_rarity: "uncommon"
+
+  # ==================== PLATE (WARRIOR ARMOR) ====================
+  scale_mail:
+    template_id: "scale_mail"
+    name: "Scale Mail"
+    item_type: "armor"
+    description: "Overlapping metal scales on leather backing"
+    base_defense: 12
+    base_resistance: 2
+    base_value: 60
+    required_level: 4
+    drop_weight: 0.8
+
+  half_plate:
+    template_id: "half_plate"
+    name: "Half Plate"
+    item_type: "armor"
+    description: "Plate armor protecting vital areas"
+    base_defense: 16
+    base_resistance: 2
+    base_value: 120
+    required_level: 6
+    drop_weight: 0.5
+    min_rarity: "rare"
+
+  plate_armor:
+    template_id: "plate_armor"
+    name: "Plate Armor"
+    item_type: "armor"
+    description: "Full metal plate protection"
+    base_defense: 22
+    base_resistance: 3
+    base_value: 200
+    required_level: 7
+    drop_weight: 0.4
+    min_rarity: "rare"