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

Hi @opensolon,

Love the work on opensolon/solon! 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:

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!

The issue names no Solon files, tests, or entry points. Start by reviewing Solon's Java plugin and API extension points, then compare them with the Python-only neuforge-pay example; the scope, compatibility approach, budget semantics, and completion tests need agreement before implementation can begin.