fix(prisma): add retry for Aurora Serverless v2 connection errors

[konokenj]

Issue

close #104
close #105

Problem

The starter kit has three issues with Prisma + Aurora Serverless v2 (auto-pause enabled with minCapacity: 0):

1. Credential leak: console.log(process.env.DATABASE_URL) in prisma.ts outputs the full connection string including password to CloudWatch Logs.

2. No runtime retry: Aurora drops idle connections after idle_session_timeout (60s) and takes ~15s to resume from auto-pause (docs). Without retry, queries fail with transient errors (P1017, ECONNRESET) and do not recover.

3. No migration retry: migration-runner.ts runs prisma db push without retry. During cdk deploy, Aurora may still be resuming, causing P1001 ("Can't reach database server") and failing the entire deployment.

Solution

• Remove console.log(DATABASE_URL) to fix the credential leak.
• Add a Prisma client extension (Prisma.defineExtension with $allModels.$allOperations) that retries transient connection errors with exponential backoff. Retryable errors: P2024, P1001, P1017, idle-session timeout, ECONNRESET. Non-retryable errors (auth failures, schema errors) are thrown immediately.
• Add retry to migration-runner.ts for prisma db push with exponential backoff (base 3s, max 5 attempts, ~100s worst case within Lambda 5min timeout). Only P1001 / connection refused are retried.
• Optimize connection parameters: connection_limit=1 (Lambda handles one request per instance), connect_timeout=30 (accommodates auto-pause resume time).

Changes

• webapp/src/lib/prisma.ts — Remove console.log, remove verbose log option, add retry extension via $extends
• webapp/src/jobs/migration-runner.ts — Extract runPrismaDbPush with retry loop, structured logging
• cdk/lib/constructs/database.ts — Change connection options to ?connection_limit=1&connect_timeout=30

Verification

• console.log(process.env.DATABASE_URL) is removed
• After Aurora auto-pause resume, the first request recovers via retry
• Non-retryable errors (e.g. auth failure) are thrown immediately without retry
• cdk deploy succeeds even when Aurora is resuming from 0 ACU
• tsc --noEmit passes
• prettier --check passes