feat(oracle): add price deviation safety check worker

[phertyameen]

Summary

Implements an automated safety guard that continuously compares stored
oracle prices against CoinGecko reference prices. If any asset deviates
beyond the configured threshold, signing is immediately halted and an
alert is emitted.

How it works

1. A @Cron worker fires every 60 s (configurable via env)
2. Fetches the latest stored oracle price per symbol from the DB
3. Calls CoinGecko's /simple/price endpoint (single batched request)
4. Computes percentage deviation for each symbol
5. If any symbol exceeds the threshold - halts signing and fires a
   structured error log for downstream alerting (PagerDuty, Slack, etc.)
6. Every check (breach or not) is persisted to price_deviation_log

New files

• price-deviation.config.ts - threshold %, cron expression, symbol→CoinGecko ID map
• price-deviation-log.entity.ts - TypeORM audit log entity
• coingecko.service.ts - batched CoinGecko HTTP wrapper
• price-deviation.service.ts - deviation logic, halt flag, alert emission
• price-deviation.worker.ts - @Cron scheduler driving the checks
• price-deviation.module.ts - wires everything together
• price-deviation.service.spec.ts - unit tests

Environment variables

Variable	Default	Description
PRICE_DEVIATION_THRESHOLD_PERCENT	5	Max allowed % deviation
PRICE_DEVIATION_CRON	*/60 * * * * *	Worker schedule

Integration steps

• Import PriceDeviationModule in AppModule
• Add PriceDeviationLog to TypeORM entities array
• Register your OraclePrice entity in the module
• Inject PriceDeviationService into your signing service and guard
  on isSigningHalted()
• Wire clearHalt() to an admin/ops endpoint for manual recovery
• Configure logger transport to forward PRICE_DEVIATION_BREACH
  alerts to your alerting stack

Testing

• Unit tests pass (npm run test)
• Manually trigger a breach by setting threshold to 0 and verify
  signing halts
• Verify price_deviation_log rows are written each cycle

Closes #119

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