feat: add per-endpoint cache TTL configuration via env vars (#282)

[Ebenezer199914]

Summary

Resolves #282 — Performance & Caching | Add per-endpoint cache TTL configuration via environment variables.

Previously, all cache TTLs were hardcoded constants scattered across individual route files, with only a single CACHE_TTL_MS env var controlling the fee-estimate endpoint.

Changes

New file: src/config/cacheConfig.js

Centralised TTL config module. Reads six dedicated env vars (in ms), converts to seconds, and exports a plain cacheTTL object used by all route handlers.

Env var	Endpoint	Default
CACHE_TTL_NETWORK_STATUS_MS	/network-status	5 000 ms
CACHE_TTL_FEE_ESTIMATE_MS	/fee-estimate + surge-status	5 000 ms
CACHE_TTL_BASE_FEE_MS	/network/base-fee	5 000 ms
CACHE_TTL_VALIDATORS_MS	/network/validators	300 000 ms
CACHE_TTL_ASSET_MS	/asset/:code/:issuer	30 000 ms
CACHE_TTL_ASSET_PRICE_MS	asset price endpoint	5 000 ms

The legacy CACHE_TTL_MS variable is still honoured as a global fallback — existing deployments are unaffected.

Updated routes

• src/routes/networkStatus.js — removed hardcoded CACHE_TTL = 5
• src/routes/feeEstimate.js — removed inline env-var derivation
• src/routes/network.js — removed VALIDATORS_CACHE_TTL and BASE_FEE_CACHE_TTL constants
• src/routes/asset.js — removed hardcoded 30 and CACHE_TTL_ASSET_PRICE constant

.env.example

Added all six per-endpoint variables with inline documentation alongside the existing CACHE_TTL_MS global fallback.

Tests: tests/cacheConfig.test.js

20 unit tests covering:

• Default values for all 6 keys
• Per-endpoint env var overrides
• Global CACHE_TTL_MS fallback precedence
• Edge cases: NaN, zero, negative, large values

Testing

PASS tests/cacheConfig.test.js        (20/20)
PASS tests/cache.service.test.js
PASS tests/cache.integration.test.js
PASS tests/feeEstimate.test.js
PASS tests/network.base-fee.test.js
PASS tests/network.validators.test.js
PASS tests/asset.test.js

Tests: 45 passed, 45 total

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