feat(combat): add combat domain enums, entities, and encounter consumption field

apps/api/src/combat/combat-action.enum.ts

@@ -0,0 +1,6 @@
+// Only ATTACK is implemented in Slice 0.3. Future slices add HEAVY_STRIKE,
+// SHIELD_BASH, DEFEND, POTION, FLEE as real members with their own
+// CombatEngineService cases — do not add them here until their behavior ships.
+export enum CombatAction {
+  ATTACK = 'ATTACK',
+}