feat(exchange): add MEXC paper trading

[katsushi2441]

Summary

• Problem: NOFX requires real exchange credentials and funds before users can safely evaluate an AI strategy against live market prices.
• What changed: Added an API-key-free MEXC paper exchange that reads public MEXC Spot V3 prices/candles and simulates USDT-margined long/short positions, leverage, fees, stop-loss, take-profit, balances, orders, and closed PnL locally. Added API routing, exchange setup UI, persistent paper accounts, MEXC interval normalization/aggregation, and tests.
• What did NOT change (scope boundary): No MEXC private or order endpoint is called. Existing live exchange behavior, credentials, and order execution are unchanged.

Change Type

• Bug fix
• Feature
• Refactoring
• Docs
• Security fix
• Chore / infra

Scope

• Trading engine / strategies
• MCP / AI clients
• API / server
• Telegram bot / agent
• Web UI / frontend
• Config / deployment
• CI/CD / infra

Linked Issues

• Related: none

Testing

What you verified and how:

• go build ./... passes
• go test ./... passes
• Manual testing done (describe below)

Manual verification:

• Confirmed MEXC public ticker, symbol, native 1h, and aligned aggregated 3m candle retrieval.
• Confirmed a 10,000 USDT paper account can start without credentials.
• Confirmed long/short fills, fees, persistence, insufficient-margin rejection, and stop-loss/take-profit execution through automated tests.
• npm test -- --run: 128 tests passed.
• npm run build: TypeScript and Vite production build passed.

Security Impact

• Secrets/keys handling changed? No; MEXC Paper neither accepts nor requires exchange credentials.
• New/changed API endpoints? No new paths; existing exchange, symbol, candle, account, and trader endpoints now support mexc_paper.
• User input validation affected? Yes; mexc_paper is accepted as a credential-free exchange type, while existing exchange validation is unchanged.

Compatibility

• Backward compatible? Yes
• Config/env changes? No; NOFX_MEXC_PAPER_DATA_DIR is optional and defaults to data/mexc-paper.
• Migration needed? No
• If yes, upgrade steps: N/A