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/affixes/prefixes.yaml

@@ -0,0 +1,177 @@
+# Item Prefix Affixes
+# Prefixes appear before the item name: "Flaming Dagger"
+#
+# Affix Structure:
+#   affix_id: Unique identifier
+#   name: Display name (what appears in the item name)
+#   affix_type: "prefix"
+#   tier: "minor" (RARE), "major" (EPIC), "legendary" (LEGENDARY only)
+#   description: Flavor text describing the effect
+#   stat_bonuses: Dict of stat_name -> bonus value
+#   defense_bonus: Direct defense bonus
+#   resistance_bonus: Direct resistance bonus
+#   damage_bonus: Flat damage bonus (weapons)
+#   damage_type: Elemental damage type
+#   elemental_ratio: Portion converted to elemental (0.0-1.0)
+#   crit_chance_bonus: Added to crit chance
+#   crit_multiplier_bonus: Added to crit multiplier
+#   allowed_item_types: [] = all types, or ["weapon", "armor"]
+#   required_rarity: null = any, or "legendary"
+
+prefixes:
+  # ==================== ELEMENTAL PREFIXES (FIRE) ====================
+  flaming:
+    affix_id: "flaming"
+    name: "Flaming"
+    affix_type: "prefix"
+    tier: "minor"
+    description: "Imbued with fire magic, dealing bonus fire damage"
+    damage_type: "fire"
+    elemental_ratio: 0.25
+    damage_bonus: 3
+    allowed_item_types: ["weapon"]
+
+  blazing:
+    affix_id: "blazing"
+    name: "Blazing"
+    affix_type: "prefix"
+    tier: "major"
+    description: "Wreathed in intense flames"
+    damage_type: "fire"
+    elemental_ratio: 0.35
+    damage_bonus: 6
+    allowed_item_types: ["weapon"]
+
+  # ==================== ELEMENTAL PREFIXES (ICE) ====================
+  frozen:
+    affix_id: "frozen"
+    name: "Frozen"
+    affix_type: "prefix"
+    tier: "minor"
+    description: "Enchanted with frost magic"
+    damage_type: "ice"
+    elemental_ratio: 0.25
+    damage_bonus: 3
+    allowed_item_types: ["weapon"]
+
+  glacial:
+    affix_id: "glacial"
+    name: "Glacial"
+    affix_type: "prefix"
+    tier: "major"
+    description: "Encased in eternal ice"
+    damage_type: "ice"
+    elemental_ratio: 0.35
+    damage_bonus: 6
+    allowed_item_types: ["weapon"]
+
+  # ==================== ELEMENTAL PREFIXES (LIGHTNING) ====================
+  shocking:
+    affix_id: "shocking"
+    name: "Shocking"
+    affix_type: "prefix"
+    tier: "minor"
+    description: "Crackles with electrical energy"
+    damage_type: "lightning"
+    elemental_ratio: 0.25
+    damage_bonus: 3
+    allowed_item_types: ["weapon"]
+
+  thundering:
+    affix_id: "thundering"
+    name: "Thundering"
+    affix_type: "prefix"
+    tier: "major"
+    description: "Charged with the power of storms"
+    damage_type: "lightning"
+    elemental_ratio: 0.35
+    damage_bonus: 6
+    allowed_item_types: ["weapon"]
+
+  # ==================== MATERIAL PREFIXES ====================
+  iron:
+    affix_id: "iron"
+    name: "Iron"
+    affix_type: "prefix"
+    tier: "minor"
+    description: "Reinforced with sturdy iron"
+    stat_bonuses:
+      constitution: 1
+    defense_bonus: 2
+
+  steel:
+    affix_id: "steel"
+    name: "Steel"
+    affix_type: "prefix"
+    tier: "major"
+    description: "Forged from fine steel"
+    stat_bonuses:
+      constitution: 2
+      strength: 1
+    defense_bonus: 4
+
+  # ==================== QUALITY PREFIXES ====================
+  sharp:
+    affix_id: "sharp"
+    name: "Sharp"
+    affix_type: "prefix"
+    tier: "minor"
+    description: "Honed to a fine edge"
+    damage_bonus: 3
+    crit_chance_bonus: 0.02
+    allowed_item_types: ["weapon"]
+
+  keen:
+    affix_id: "keen"
+    name: "Keen"
+    affix_type: "prefix"
+    tier: "major"
+    description: "Razor-sharp edge that finds weak points"
+    damage_bonus: 5
+    crit_chance_bonus: 0.04
+    allowed_item_types: ["weapon"]
+
+  # ==================== DEFENSIVE PREFIXES ====================
+  sturdy:
+    affix_id: "sturdy"
+    name: "Sturdy"
+    affix_type: "prefix"
+    tier: "minor"
+    description: "Built to withstand punishment"
+    defense_bonus: 3
+    allowed_item_types: ["armor"]
+
+  reinforced:
+    affix_id: "reinforced"
+    name: "Reinforced"
+    affix_type: "prefix"
+    tier: "major"
+    description: "Heavily reinforced for maximum protection"
+    defense_bonus: 5
+    resistance_bonus: 2
+    allowed_item_types: ["armor"]
+
+  # ==================== LEGENDARY PREFIXES ====================
+  infernal:
+    affix_id: "infernal"
+    name: "Infernal"
+    affix_type: "prefix"
+    tier: "legendary"
+    description: "Burns with hellfire"
+    damage_type: "fire"
+    elemental_ratio: 0.45
+    damage_bonus: 12
+    allowed_item_types: ["weapon"]
+    required_rarity: "legendary"
+
+  vorpal:
+    affix_id: "vorpal"
+    name: "Vorpal"
+    affix_type: "prefix"
+    tier: "legendary"
+    description: "Cuts through anything with supernatural precision"
+    damage_bonus: 10
+    crit_chance_bonus: 0.08
+    crit_multiplier_bonus: 0.5
+    allowed_item_types: ["weapon"]
+    required_rarity: "legendary"