fix: preserve runestone bounty flag for legacy defeated bosses (#67)

## Summary

- Bosses defeated before `bountyRunestonesClaimed` was introduced had `status: "defeated"` but the field `undefined`
- After prestige, the preservation check (`=== true`) missed these bosses, so the first-kill bounty was re-awarded on the next run
- Now also treats `status === "defeated"` as proof the bounty was already earned, covering the migration case

## Test plan

- [ ] Existing test: `preserves bountyRunestonesClaimed flag on bosses across prestige` — still passes
- [ ] New test: `sets bountyRunestonesClaimed on bosses defeated before the flag was introduced` — covers the legacy save migration path
- [ ] Full coverage maintained at 100%

Closes #52

✨ This PR was created with help from Hikari~ 🌸

Reviewed-on: #67
Co-authored-by: Hikari <hikari@nhcarrigan.com>
Co-committed-by: Hikari <hikari@nhcarrigan.com>

apps/api/src/services/prestige.ts

@@ -215,7 +215,10 @@ const buildPostPrestigeState = (
     const currentBoss = currentState.bosses.find((candidate) => {
       return candidate.id === freshBoss.id;
     });
-    if (currentBoss?.bountyRunestonesClaimed === true) {
+    if (
+      currentBoss?.bountyRunestonesClaimed === true
+      || currentBoss?.status === "defeated"
+    ) {
       return { ...freshBoss, bountyRunestonesClaimed: true };
     }
     return freshBoss;

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",