feat: add GET /pegs route for comparative peg deviation monitoring

[decrypted-dev]

Closes #287

Summary of Changes
Config & Types (src/config.ts):
Added PeggedAssetConfig interface (asset, target, targetPrice, windowHours).
Added parsing and resolution logic for PEGGED_ASSETS_TESTNET, PEGGED_ASSETS_MAINNET, and fallback PEGGED_ASSETS.
Explicit target and peg target prices are configured without guessing from tickers.
Preserves complete 56-character contract IDs / issuer addresses.
GET /pegs Route (src/routes/pegs.ts):
Queries and returns all configured pegged assets in a single response.
Calculates currentPrice, currentDeviationBps, and worstDeviationBps over the configured window.
Unconfigured assets are absent from the response (not reported as zero).
Reports full provenance for each figure: venues, sample count, freshness in seconds, and latest sample timestamp.
Honest representation when there is no liquidity/data: returns null rather than 0 or 500.
Reads req.network ?? activeNetwork, filters queries strictly by network, and echoes network in the response.
OpenAPI Specification & Schemas (openapi.yaml, openapi.json, src/api/schemas.ts):
Declared /pegs route with ?network= parameter and detailed schema definitions (PegReport, PegsResponse).
Synced with openapi.json via the generator script.
Environment & Documentation (.env.example, README.md, .changeset):
Added PEGGED_ASSETS_TESTNET, PEGGED_ASSETS_MAINNET, and PEGGED_ASSETS to .env.example passing the env-drift guard.
Documented /pegs in README.md and created a changeset.
Testing (src/tests/pegs.test.ts):
Verified that all acceptance criteria are met, including network parameter honoring, provenance reporting, omission of unconfigured assets, and honest null-handling for illiquid pairs.
All tests passing with vitest.

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