feat: start logging token usage and subscriptions (#2)

### Explanation

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### Issue

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- [x] I have read and agree to the [Code of Conduct](https://docs.nhcarrigan.com/community/coc/)
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### Tests

- [ ] My contribution adds new code, and I have added tests to cover it.
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### Versioning

Minor - My pull request introduces a new non-breaking feature.

Reviewed-on: #2
Co-authored-by: Naomi Carrigan <commits@nhcarrigan.com>
Co-committed-by: Naomi Carrigan <commits@nhcarrigan.com>

src/modules/proofread.ts

@@ -6,6 +6,7 @@
 import { MessageFlags, type ChatInputCommandInteraction } from "discord.js";
 import { personality } from "../config/personality.js";
 import { ai } from "../utils/ai.js";
+import { calculateCost } from "../utils/calculateCost.js";
 import { isSubscribed } from "../utils/isSubscribed.js";
 
 /**
@@ -41,4 +42,7 @@ export const proofread = async(
     response?.text
       ?? "I'm sorry, I don't have an answer for that. Please try again later.",
   );
+
+  const { usage } = messages;
+  await calculateCost(usage, interaction.user.username, "proofread");
 };