fix: restore upgrade drops to late-game bosses

Assigned 3 previously orphaned adventurer upgrades to appropriate bosses:
- horizon_beast (Infinite Expanse) → oblivion_paladin_1
- maelstrom_god (Cosmic Maelstrom) → transcendent_rogue_1
- eternal_end (The Absolute) → omniversal_champion_1

Closes #140

apps/api/src/data/bosses.ts

@@ -992,7 +992,7 @@ export const defaultBosses: Array<Boss> = [
     name:                "The Horizon Beast",
     prestigeRequirement: 8,
     status:              "locked",
-    upgradeRewards:      [],
+    upgradeRewards:      [ "oblivion_paladin_1" ],
     zoneId:              "infinite_expanse",
   },
   {
@@ -1156,7 +1156,7 @@ export const defaultBosses: Array<Boss> = [
     name:                "The Maelstrom God",
     prestigeRequirement: 13,
     status:              "locked",
-    upgradeRewards:      [],
+    upgradeRewards:      [ "transcendent_rogue_1" ],
     zoneId:              "cosmic_maelstrom",
   },
   {
@@ -1302,7 +1302,7 @@ export const defaultBosses: Array<Boss> = [
     name:                "The Eternal End",
     prestigeRequirement: 19,
     status:              "locked",
-    upgradeRewards:      [],
+    upgradeRewards:      [ "omniversal_champion_1" ],
     zoneId:              "the_absolute",
   },
   {