fix: dedup logger, replace magic number, validate Stellar key format, add coverage script

[unlimitedengineer]

Closes #248
Closes #249
Closes #250
Closes #245

#248 — Replace hardcoded limit: 20 with WARM_PAGE_LIMIT (src/cache/warmer.ts)
The landingPageQuery on line 27 used a magic 20 instead of the already-defined WARM_PAGE_LIMIT constant. Now both the landing-page query and the pagination loop use the same constant.

#249 — Remove duplicate Fastify logger config (src/server.ts)
Fastify's constructor was given its own inline logger object (level + transport) that duplicated the identical fields already configured in src/utils/logger.ts. Replaced the inline config with the singleton logger import so there is exactly one place that controls log level, transport, and serializers.

#250 — Add test:coverage script (package.json)
Added "test:coverage": "vitest run --coverage" alongside the existing test and test:watch scripts. @vitest/coverage-v8 is already listed as a dev dependency.

#245 — Validate Stellar secret key format (src/config/index.ts)
STELLAR_PLATFORM_SECRET was only validated with min(1), allowing any non-empty string. Updated to regex(/^S[A-Z2-7]{55}$/) so invalid keys are rejected at startup with a clear error message.

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