fix: add env var validation warnings and centralized config defaults

[praise-idise]

Closes #1001

Summary

The backend server silently defaulted critical environment variables
to localhost/testnet values without any warning. This adds clear
warning logs, SSL tracking, and replaces scattered inline fallbacks
with the centralized getEnvVar() utility.

Problem

• REDIS_URL → redis://localhost:6379 without warning
• NODE_ENV → development without warning
• SOROBAN_RPC_URL → testnet without warning
• STELLAR_NETWORK → testnet without warning
• SSL_KEY_PATH / SSL_CERT_PATH → not tracked
• Multiple files used their own inline process.env.X || 'fallback'

What Changed

File	Change
src/utils/checkEnv.ts	warnOnDefault flag, SSL vars, HTTPS status logging
src/utils/redis.ts	Use centralized getEnvVar() instead of inline fallback
src/config/redis.config.ts	Warning log on Redis config fallback
tests/checkEnv.test.ts	3 new tests + fix pre-existing test

Startup Output Example

⚠️  NODE_ENV is not set, defaulting to "development".
⚠️  STELLAR_NETWORK is not set, defaulting to "testnet".
⚠️  SOROBAN_RPC_URL is not set, defaulting to "https://soroban-testnet.stellar.org".
⚠️  REDIS_URL is not set, defaulting to redis://localhost:6379.
ℹ️  SSL_KEY_PATH and SSL_CERT_PATH are not set. HTTPS is not configured.
✅ Environment variables validated successfully

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