feat(oracle-worker): OKX REST price fetcher

[IbrahimIjai]

Context

The oracle currently aggregates prices from Binance, Coinbase, and Pyth. Adding OKX as a fourth source improves median robustness — a single exchange outage or manipulation attempt now has to beat three other sources instead of two before it can skew the aggregated price.

OKX public ticker API requires no API key for spot prices.

Implementation

Add `oracle/src/sources/okx.rs` following the same interface pattern as the existing fetchers:

GET https://www.okx.com/api/v5/market/ticker?instId=<SYMBOL>-USDT

Response field: `data[0].last` (string, USD price).

• Map SO4 token symbols to OKX instrument IDs via the `PRICE_FEED_CONFIG` `sources` field (add `"okx"` as a valid source name)
• Parse the price string to `i128` at `FLOAT_PRECISION`
• Plug into the existing `aggregate_prices` pipeline alongside Binance, Coinbase, and Pyth

Acceptance criteria

• `oracle/src/sources/okx.rs` implemented with the same `fetch_prices(symbols) → HashMap<String, i128>` signature as other fetchers
• OKX optionally included per token via `PRICE_FEED_CONFIG` `"sources": ["okx"]`
• HTTP errors and JSON parse failures return `Err`, do not panic
• Unit test with a mocked HTTP response covering happy path and error path
• OKX prices flow through outlier rejection and median aggregation correctly