feat: add essence infusion upgrades as late-prestige essence sink (#77)

## Summary

Closes #62

Adds five **Essence Infusion** upgrades (I–V) to give essence an ongoing meaningful use deep into a prestige run, when gold upgrades are all purchased and essence reserves are in the trillions with nowhere to go:

| Upgrade | Cost | Multiplier |
|---|---|---|
| Essence Infusion I | 1T essence | ×2 global |
| Essence Infusion II | 5T essence | ×2 global |
| Essence Infusion III | 25T essence | ×2 global |
| Essence Infusion IV | 100T essence | ×3 global |
| Essence Infusion V | 500T essence | ×5 global |

All five start `unlocked: true` (no prerequisite boss or quest required) and cost zero gold and zero crystals — they are purely essence sinks. Combined, they provide a ×120 global income multiplier for players willing to pour their essence reserves into the guild. The About panel's Upgrades section is also updated to inform players these exist.

CDN art assets will need to be generated for IDs `essence_sink_1` through `essence_sink_5` in the `upgrades` folder.

✨ This issue was created with help from Hikari~ 🌸

Reviewed-on: #77
Co-authored-by: Hikari <hikari@nhcarrigan.com>
Co-committed-by: Hikari <hikari@nhcarrigan.com>

apps/api/src/data/upgrades.ts

@@ -767,4 +767,70 @@ export const defaultUpgrades: Array<Upgrade> = [
     target:     "adventurer",
     unlocked:   false,
   },
+  // ── Essence Sinks ─────────────────────────────────────────────────────────
+  {
+    costCrystals: 0,
+    costEssence:  1e12,
+    costGold:     0,
+    description:
+      "Channel a vast reservoir of essence into the guild's core — all production ×2.",
+    id:         "essence_sink_1",
+    multiplier: 2,
+    name:       "Essence Infusion I",
+    purchased:  false,
+    target:     "global",
+    unlocked:   true,
+  },
+  {
+    costCrystals: 0,
+    costEssence:  5e12,
+    costGold:     0,
+    description:
+      "A deeper infusion saturates every operation with raw essence — all production ×2.",
+    id:         "essence_sink_2",
+    multiplier: 2,
+    name:       "Essence Infusion II",
+    purchased:  false,
+    target:     "global",
+    unlocked:   true,
+  },
+  {
+    costCrystals: 0,
+    costEssence:  2.5e13,
+    costGold:     0,
+    description:
+      "Essence floods the ley-lines binding your guild — all production ×2.",
+    id:         "essence_sink_3",
+    multiplier: 2,
+    name:       "Essence Infusion III",
+    purchased:  false,
+    target:     "global",
+    unlocked:   true,
+  },
+  {
+    costCrystals: 0,
+    costEssence:  1e14,
+    costGold:     0,
+    description:
+      "The guild breathes essence as its very lifeblood — all production ×3.",
+    id:         "essence_sink_4",
+    multiplier: 3,
+    name:       "Essence Infusion IV",
+    purchased:  false,
+    target:     "global",
+    unlocked:   true,
+  },
+  {
+    costCrystals: 0,
+    costEssence:  5e14,
+    costGold:     0,
+    description:
+      "Essence transcends material form and reshapes reality itself — all production ×5.",
+    id:         "essence_sink_5",
+    multiplier: 5,
+    name:       "Essence Infusion V",
+    purchased:  false,
+    target:     "global",
+    unlocked:   true,
+  },
 ];