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

Hi @firecrawl,

Love the work on `firecrawl/firecrawl-mcp-server`! 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!

No repository file or test is named; start by reviewing the server's existing API entry points and dependency structure, then assess whether optional neuforge-pay integration fits the project. Done would require an agreed design for token metering, paid API keys, and session budget enforcement, plus documented validation.

Written by the indexing model from the issue text.