feat(api): implement inventory service with equipment system

Add InventoryService for managing character inventory, equipment, and
consumable usage. Key features:

- Add/remove items with inventory capacity checks
- Equipment slot validation (weapon, off_hand, helmet, chest, gloves,
  boots, accessory_1, accessory_2)
- Level and class requirement validation for equipment
- Consumable usage with instant and duration-based effects
- Combat-specific consumable method returning effects for combat system
- Bulk operations (add_items, get_items_by_type, get_equippable_items)

Design decision: Uses full Item object storage (not IDs) to support
procedurally generated items with unique identifiers.

Files added:
- /api/app/services/inventory_service.py (560 lines)
- /api/tests/test_inventory_service.py (51 tests passing)

Task 2.3 of Phase 4 Combat Implementation complete.

api/docs/GAME_SYSTEMS.md

@@ -402,6 +402,111 @@ effects_applied:
 
 ---
 
+## Procedural Item Generation
+
+### Overview
+
+Weapons and armor are procedurally generated using a Diablo-style affix system.
+Items are created by combining:
+1. **Base Template** - Defines item type, base stats, level requirement
+2. **Affixes** - Prefixes and suffixes that add stats and modify the name
+
+### Generation Process
+
+1. Select base template (filtered by level, rarity)
+2. Determine affix count based on rarity (0-3)
+3. Roll affix tier based on rarity weights
+4. Select random affixes avoiding duplicates
+5. Combine stats and generate name
+
+### Rarity System
+
+| Rarity | Affixes | Value Multiplier | Color |
+|--------|---------|------------------|-------|
+| COMMON | 0 | 1.0× | Gray |
+| UNCOMMON | 0 | 1.5× | Green |
+| RARE | 1 | 2.5× | Blue |
+| EPIC | 2 | 5.0× | Purple |
+| LEGENDARY | 3 | 10.0× | Orange |
+
+### Affix Distribution
+
+| Rarity | Affix Count | Distribution |
+|--------|-------------|--------------|
+| RARE | 1 | 50% prefix OR 50% suffix |
+| EPIC | 2 | 1 prefix AND 1 suffix |
+| LEGENDARY | 3 | Mix (2+1 or 1+2) |
+
+### Affix Tiers
+
+Higher rarity items have better chances at higher tier affixes:
+
+| Rarity | MINOR | MAJOR | LEGENDARY |
+|--------|-------|-------|-----------|
+| RARE | 80% | 20% | 0% |
+| EPIC | 30% | 70% | 0% |
+| LEGENDARY | 10% | 40% | 50% |
+
+### Name Generation Examples
+
+- **COMMON:** "Dagger"
+- **RARE (prefix):** "Flaming Dagger"
+- **RARE (suffix):** "Dagger of Strength"
+- **EPIC:** "Flaming Dagger of Strength"
+- **LEGENDARY:** "Blazing Glacial Dagger of the Titan"
+
+### Luck Influence
+
+Player's LUK stat affects rarity rolls for loot drops:
+
+**Base chances at LUK 8:**
+- COMMON: 50%
+- UNCOMMON: 30%
+- RARE: 15%
+- EPIC: 4%
+- LEGENDARY: 1%
+
+**Luck Bonus:**
+Each point of LUK above 8 adds +0.5% to higher rarity chances.
+
+**Examples:**
+- LUK 8 (baseline): 1% legendary chance
+- LUK 12: ~3% legendary chance
+- LUK 16: ~5% legendary chance
+
+### Service Usage
+
+```python
+from app.services.item_generator import get_item_generator
+from app.models.enums import ItemRarity
+
+generator = get_item_generator()
+
+# Generate item of specific rarity
+sword = generator.generate_item(
+    item_type="weapon",
+    rarity=ItemRarity.EPIC,
+    character_level=5
+)
+
+# Generate random loot with luck bonus
+loot = generator.generate_loot_drop(
+    character_level=10,
+    luck_stat=15
+)
+```
+
+### Data Files
+
+| File | Description |
+|------|-------------|
+| `/app/data/base_items/weapons.yaml` | 13 weapon templates |
+| `/app/data/base_items/armor.yaml` | 12 armor templates |
+| `/app/data/affixes/prefixes.yaml` | 18 prefix affixes |
+| `/app/data/affixes/suffixes.yaml` | 11 suffix affixes |
+
+---
+
 ## Quest System (Future)
 
 ### Quest Types