feat: predictions list per market

[Bellamzy]

feat: cursor-paginated predictions list per market

Summary

Adds GET /api/v1/markets/:id/predictions — a cursor-paginated endpoint that returns all
predictions placed in a specific market (pool), ordered newest-first.

Changes

backend/src/db.rs

• MarketPredictionRow — typed row struct for the predictions table
• get_market_predictions — cursor query using id DESC + limit+1 trick to detect next-page
  without an extra COUNT
• count_market_predictions — total count for the response envelope

backend/src/routes/v1.rs

• MarketPredictionsQuery — ?after=&limit=, limit clamped 1–100, default 20
• get_market_predictions handler — 404 guard for unknown markets, next_cursor derived from the
  extra row
• Route registered: GET /markets/{id}/predictions

backend/src/openapi.rs

• MarketPredictionDoc + MarketPredictionsResponse schemas
• api_get_market_predictions path stub with 200 / 404 / 503 responses

backend/README.md

• Endpoint reference with pagination example

Pagination

Pass next_cursor from the response as ?after= on the next request. A null cursor means
the last page has been reached.

GET /api/v1/markets/42/predictions?limit=20
GET /api/v1/markets/42/predictions?limit=20&after=302

Tests

• 4 route-level tests: no-db guard, bad market ID → 422, oversized limit clamped, lazy-db
  wiring
• 5 unit tests: cursor/next_cursor logic, empty result, single-row edge case, clamp boundaries

Testing

cd backend
cargo test
closes #171

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[Yunusabdul38]

LGTM
Thanks for your contribution