feat: add order book depth, fx indexer, investor scoring, and announcements endpoints

[Omoboi-dev]

Summary of Changes

This pull request implements the requested endpoints, route handlers, data contracts, and unit tests across secondary market visualization, oracle FX rates, investor reputation scoring, and issuer announcement feeds:

1. Order Book Depth Endpoint (Build order book depth API endpoint for secondary market visualisation #664):
   • Added GET /invoices/:id/orderbook (mounted on /api/v1/invoices and /invoices) returning bids and asks grouped by price level with price, cumulative amounts, order counts, and a 15-second cache header.
2. FX Rate Indexer & Conversion API (Implement FX rate indexer and currency conversion API #663):
   • Implemented createFxRouter mounted at /api/v1/fx and /fx.
   • Exposes GET /fx/rates returning all currency pairs with staleness indicators and GET /fx/rates/:pair returning single-pair rate details with a 60-second cache header.
3. Investor Scoring & Tier REST API (Implement investor scoring indexer and tier REST API #660):
   • Implemented GET /investors/:wallet/score returning score, tier, and last updated timestamp with 60-second cache headers.
   • Implemented GET /investors/:wallet/score/history returning cursor-paginated score change events with delta.
4. Issuer Announcement Indexer & Feed API (Add issuer announcement indexer and feed REST API #661):
   • Implemented GET /invoices/:id/announcements returning cursor-paginated announcement summaries per invoice.
   • Implemented GET /invoices/announcements/:announcementId/content returning full content with caching indicators.
5. Types & Unit Tests:
   • Added TypeScript interfaces in src/types/market.ts.
   • Added comprehensive unit tests in tests/unit/market-features.test.ts.

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Closes #660
Closes #661
Closes #663
Closes #664

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