feat: add /basket endpoint for weighted multi-asset basket pricing

[collinsezedike]

Closes #97

Summary

• Adds GET /basket?asset=...&weight=... accepting repeated query params for multi-asset baskets
• Normalizes supplied weights to sum to 1.0 before computing the basket price, regardless of raw input
• Fetches the 5-minute VWAP per asset from price_points (same pattern as the oracle comparison route)
• Returns a full breakdown per component (asset, price, normalized weight, weighted contribution)
• Validates: minimum 2 assets, matching asset/weight array lengths, all weights positive, and price data availability

New files

• src/routes/basket.ts: route handler
• src/__tests__/basket.test.ts: unit tests covering happy path, weight normalization, and all validation/error branches

Test plan

• Equal weights produce a 50/50 split and correct basket price
• Unequal raw weights (e.g., 3:1) normalize to 0.75/0.25 and basket price matches
• Single asset returns 400
• Mismatched asset/weight counts return 400
• Zero or negative weight returns 400
• Asset with no recent price data returns 404

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