feat: implement stablecoin peg deviation benchmark route

[barnabasolutayo-lgtm]

##closes #98

# Description

Implements a new route `/benchmark/:asset?target=USD` to track stablecoin peg deviations. The endpoint calculates deviation in basis points and computes rolling 24-hour statistics.

## Key Features

- **Dynamic Pair Resolution**: Resolves the matching watched pair dynamically using `getActivePairs()` from the active registry.
- **Directional Precision**: Automatically checks if the queried asset is the base or counter asset (i.e. `assetA` or `assetB`) in the pair configuration and applies the inverse mathematical operation (`1 / price`) when required.
- **Basis Points (BP) Precision**: Computes deviation using `(Price - 1.0) * 10000`.
- **24-Hour Rolling Statistics**: Performs a single database query to extract:
  - Overall latest price
  - 24h rolling min deviation
  - 24h rolling max deviation
  - 24h rolling max absolute deviation
  - 24h rolling average deviation
  - 24h rolling sample count
- **Strict Response Validation**: Registers a custom Fastify schema `benchmarkResponseSchema` to validate the route payload structure and prevent regression in dev/test setups.

## Files Modified / Added

- **[NEW]** `src/routes/benchmark.ts`: Main route handler and math calculation layer.
- **[NEW]** `src/__tests__/benchmark.test.ts`: Unit tests validating error responses, forward/inverse math calculations, and empty db cases.
- **[MODIFY]** `src/api/schemas.ts`: Added `benchmarkResponseSchema` for response validation.
- **[MODIFY]** `src/index.ts`: Registered the benchmark routes.

## Test Verification

Verified using Vitest:
- Added and ran `src/__tests__/benchmark.test.ts`.
- Verified that all unit tests pass (149/149).

```bash
npx vitest run src/__tests__/benchmark.test.ts

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