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;