feat: add multi-ID delete support with per-ID results

- Add delete_bounties method to BountyService that returns individual
  results per bounty ID (deleted, not_found, permission_denied)
- Update cmd_delete to accept multiple IDs and show per-ID messages
- Add tests for delete_bounties

Example output:
/delete 1 2 3
✅ Bounty #1 deleted.
✅ Bounty #2 deleted.
⛔ Bounty #3 not found.

Fixes #47

core/services.py

@@ -210,6 +210,31 @@ class BountyService:
         self._storage.update_bounty(room_id, bounty)
         return True
 
+    def delete_bounties(
+        self, room_id: int, bounty_ids: list[int], user_id: int
+    ) -> dict[int, str]:
+        """Soft delete multiple bounties. Only admins can delete.
+
+        Returns a dict mapping bounty_id to result:
+        - "deleted": Successfully soft-deleted
+        - "not_found": Bounty does not exist
+        - "permission_denied": User is not admin
+        """
+        results = {}
+        for bounty_id in bounty_ids:
+            bounty = self._storage.get_bounty(room_id, bounty_id)
+            if not bounty:
+                results[bounty_id] = "not_found"
+                continue
+            if not self.is_admin(room_id, user_id):
+                results[bounty_id] = "permission_denied"
+                continue
+
+            bounty.deleted_at = int(time.time())
+            self._storage.update_bounty(room_id, bounty)
+            results[bounty_id] = "deleted"
+        return results
+
 
 class TrackingService:
     """Service for tracking bounty operations."""