Add /usage endpoint with token tracking, live balances, and cost calculations

CONTEXT.md

@@ -235,6 +235,9 @@ sudo journalctl -u ein-ai-proxy -f --no-pager
 # Check proxy health
 curl https://ai.ein-softworks.com/health
 
+# Check usage statistics
+curl https://ai.ein-softworks.com/usage
+
 # Pull and deploy changes
 cd ~/ai-proxy && git pull
 sudo systemctl restart ein-ai-proxy
@@ -268,6 +271,15 @@ GLM-4.7's thinking block signatures are different from Anthropic's and will be r
 ### Zero Dependencies
 The proxy has zero npm dependencies — it uses only Node.js built-ins (`node:http`, `node:fs`, etc.). Do not add dependencies unless absolutely necessary.
 
+### Usage Tracking
+The proxy automatically tracks token usage for all requests:
+- Token counts are extracted from both streaming and non-streaming responses
+- Data is persisted to `usage-data.json` (gitignored)
+- Monthly totals reset on the 1st of each month
+- Costs are calculated based on pricing in `config.json`
+- Live account balances are fetched from providers that support it (DeepSeek)
+- Use `curl https://ai.ein-softworks.com/usage` to view all usage data
+
 ## Key Reference URLs
 - **Proxy API**: https://ai.ein-softworks.com
 - **Gitea**: https://git.ein-softworks.com