Feat/screener liquidity definition

[decrypted-dev]

Closes #283

Summary

Defines liquidity in the OpenAPI spec and route documentation, validates that the underlying query calculation matches the definition, and names the unit in the response.

Details & Specification

• Definition: For each watched pair on the selected network, liquidity represents total AMM reserves denominated in the quote/counter asset (asset_b), computed across all active liquidity pools (source = 'AMM') as:
  $$\sum (\text{reserve_a} \times \text{spot_price} + \text{reserve_b})$$
  evaluated at each pool's latest snapshot spot price.
• Unit: Denominated in the quote asset (asset_b), exposed explicitly as liquidity_unit on each row (e.g., USDC for XLM/USDC).
• Network Scoping: Echoes network in the top-level /screener response and documents ?network= in openapi.yaml and openapi.json.
• Number Changes: No numeric change was required to the stored calculation, as (ps.reserve_a * ps.spot_price + ps.reserve_b) already faithfully evaluates total liquidity in units of asset_b.

Verification

• npm test src/__tests__/screener.test.ts: Passes all 18 tests, including tests verifying the SQL formula structure and liquidity_unit naming.
• npm test tests/openapi.test.ts: Passes and verifies OpenAPI parity.
• npm test: Full test suite passes (631 passed).

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