Refactor: unified combat engine for PvE/PvP

- Create api/services/combat_engine.py with all shared combat logic
- Rewrite combat.py from 2820 to ~600 lines (thin orchestration)
- Fix buff consumption: fortify, berserker_rage, evade, foresight, iron_skin now actually work
- Fix stun: PvE skills now write stun to npc_status_effects
- Fix skill damage: now uses stats.attack_power consistently (includes perks)
- Fix PvPCombatActionRequest: add skill_id field for proper PvP skill support
- Remove dead code: PvP skill/item blocks copy-pasted into PvE endpoint
- Update game_logic.npc_attack to check buff modifiers (dodge, damage reduction, etc.)

api/services/models.py

@@ -89,12 +89,13 @@ class PvPCombatInitiateRequest(BaseModel):
 
 
 class PvPAcknowledgeRequest(BaseModel):
-    pass  # No body needed
+    combat_id: int
 
 
 class PvPCombatActionRequest(BaseModel):
-    action: str  # 'attack', 'defend', 'flee', 'use_item'
+    action: str  # 'attack', 'skill', 'flee', 'use_item'
     item_id: Optional[str] = None  # For use_item action
+    skill_id: Optional[str] = None  # For skill action
 
 
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