feat(combat): wire CombatModule into the application

Provide and export CharacterCombatStatsService from CharactersModule,
create CombatModule (registering the combat entities, controllers,
CombatService and CombatEngineService, and importing TravelModule and
CharactersModule), and register CombatModule in AppModule so the three
combat endpoints (attack, get combat, post action) are reachable from
the running app.

apps/api/src/app.module.ts

@@ -1,5 +1,6 @@
 import { Module } from '@nestjs/common';
 import { CharactersModule } from './characters/characters.module';
+import { CombatModule } from './combat/combat.module';
 import { DatabaseModule } from './database/database.module';
 import { HealthModule } from './health/health.module';
 import { HuntingModule } from './hunting/hunting.module';
@@ -14,6 +15,7 @@ import { WorldModule } from './world/world.module';
     TravelModule,
     WorldModule,
     HuntingModule,
+    CombatModule,
   ],
 })
 export class AppModule {}