fix: preserve bounty flag for bosses defeated before flag was introduced

Bosses killed before `bountyRunestonesClaimed` was added to the codebase
had `status: "defeated"` but the field undefined. After prestige, the
preservation check (`=== true`) missed these, so the bounty was re-awarded
next run. Now also treat `status === "defeated"` as proof the bounty was
already earned.

Closes #52

apps/api/test/services/prestige.spec.ts

@@ -319,6 +319,32 @@ describe("buildPostPrestigeState", () => {
     expect(matchingBoss?.bountyRunestonesClaimed).toBe(true);
   });
 
+  it("sets bountyRunestonesClaimed on bosses defeated before the flag was introduced", () => {
+    const legacyDefeatedBoss = {
+      bountyRunestones:    5,
+      crystalReward:       0,
+      currentHp:           0,
+      damagePerSecond:     10,
+      description:         "A boss",
+      equipmentRewards:    [] as string[],
+      essenceReward:       0,
+      goldReward:          100,
+      id:                  "troll_king",
+      maxHp:               100,
+      name:                "Troll King",
+      prestigeRequirement: 0,
+      status:              "defeated" as const,
+      upgradeRewards:      [] as string[],
+      zoneId:              "verdant_vale",
+    };
+    const state = makeMinimalState({ bosses: [ legacyDefeatedBoss ] as GameState["bosses"] });
+    const { prestigeState } = buildPostPrestigeState(state, "Tester");
+    const matchingBoss = prestigeState.bosses.find((boss) => {
+      return boss.id === "troll_king";
+    });
+    expect(matchingBoss?.bountyRunestonesClaimed).toBe(true);
+  });
+
   it("accumulates completed quests into lifetime total", () => {
     const quest = {
       id:          "q_1",