fix: Prevent cold-path JVM/Hibernate latency after extended idle periods

[Copilot]

After 3.5+ hours of zero traffic, first requests to GET /api/cards spiked to 3120ms avg / 4681ms max (baseline <500ms) due to stacked cold-path costs: stale HikariCP connections, cold Hibernate query plan cache, and JIT deoptimization.

Changes

application.yml

• show-sql: false — removes per-query SQL logging overhead (was true)
• HikariCP keep-alive — prevents connection pool going stale during idle:

  hikari:
    connection-test-query: SELECT 1
    keepalive-time: 300000ms   # heartbeat every 5 min
    idle-timeout: 600000ms     # evict connections idle >10 min

• warmup.* properties — externalizes scheduler delays for per-environment tuning

WarmupService.java (new)

• @Scheduled bean that calls getAllCreditCards() every 5 minutes (1-min initial delay), keeping JIT hot, Hibernate query plan cache warm, and HikariCP pool active between real traffic bursts
• Delays driven by warmup.fixed-delay / warmup.initial-delay properties
• Exceptions are caught and logged as warnings — warmup failure never affects request path

ThreeRiversBankApplication.java

• Added @EnableScheduling to activate the warmup task

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