feat(backend): add GET /policies and GET /policies/:holder/:policy_id…

[Petah1]

Summary

Implements paginated policy list and single-policy detail endpoints backed by an indexed in-memory store.

Changes

• `GET /policies` with filters (`status`, `holder`), cursor pagination, rate limiting
• `GET /policies/:holder/:policy_id` with 404/400 handling
• Cursor-based pagination helper (reusable across endpoints)
• Batch claim fetching — no N+1
• Response DTOs with coverage summary, expiry countdown, premium currency metadata, claim links
• All token amounts as strings (7 decimals), no floats
• Sliding-window rate limiter (60 req/min, zero new deps)
• OpenAPI 3.1 spec at `GET /openapi.json`
• 20 tests covering filters, pagination edge cases, cursor errors, concurrent inserts

Closes #40

[drips-wave]

@Petah1 Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits