Feature Suggestion: Optional token metering & paid API key support via `neuforge-pay`

Hi @antvis,

Love the work on `antvis/mcp-server-chart`! As usage grows across AI agent frameworks, server compute costs can start adding up quickly. We've also seen developers report that their autonomous agents accidentally burn through thousands of dollars in a single session because traditional payment rails lack built-in agent guardrails.

Would you be open to adding an optional usage metering & billing decorator using `neuforge-pay`? It acts as a proactive **Spend Firewall** and Merchant of Record for MCP servers. In 3 lines of code, it meters tokens, calculates live LLM COGS, and proactively blocks agent transactions that exceed a strict session budget limit (to prevent surprise bills).

Example Integration:
```python
from neuforge_pay import meter_endpoint

@app.get("/v1/query")
@meter_endpoint(price_charged_usd=0.05, model_name="claude-3-5-sonnet", session_budget_usd=10.00)
async def query_endpoint():
...
```

Happy to submit a clean PR if this aligns with your roadmap!

Start by reviewing the server's endpoint definitions and dependency configuration, then assess how the proposed neuforge_pay meter_endpoint would fit the example /v1/query route. Done would require an agreed optional integration covering usage metering, paid API keys, and session budget enforcement, but the issue does not define the project's expected billing flow or tests.

Written by the indexing model from the issue text.