feat: implement studio debt financing bond market and credit rating system

[Babin123456]

@SRV30

Related Issue

Closes #422

Summary

Adds a Studio Debt Financing Bond Market and Credit Rating System enabling studios to issue corporate bonds, take credit lines, receive credit rating evaluations (AAA to CCC), and process weekly coupon payments.

Changes Made

• Created StudioFinanceBond.js schema tracking principal, interest yield, and credit ratings.
• Created bondMarketEngine.js providing credit evaluation math and coupon processing.
• Created bondMarketController.js and bondMarketRoutes.js API endpoints.
• Registered /api/bond-market in Express app.js.
• Created bondMarketEngine.test.js validating credit rating assignments.

Testing

• Executed node --test tests/bondMarketEngine.test.js -> 100% pass rate.

Impact

Provides realistic capital market mechanics for financing high-budget film lineups.

Checklist

• Code follows project standards
• Tested locally
• No unrelated changes included
• Responsive design verified
• Accessibility considered

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