feat(scripts): migrate keeper/indexer/health-check/alert-failed-charges to shared logger

[divinemike019]

Summary

Migrates the four core operational scripts to the shared logger.ts structured logger, replacing all ad-hoc console.log/console.info patterns with leveled JSON-capable logging that respects LOG_LEVEL.

Changes

scripts/keeper.ts

• Replace local log(dryRun, msg) helper with rootLogger.child({ script: 'keeper', contract: CONTRACT_ID, rpc: RPC_URL })
• All call sites updated to logger.debug/info/warn/error with structured context fields
• LOG_LEVEL env var now respected via the shared logger
• console.error kept only in validateEnv() fatal bootstrap (pre-logger-init)
• console.log in showHelp() is intentional CLI help-text output to stdout (not a log line)

scripts/indexer.ts

• Replace local log(level, msg) + hand-rolled LOG_LEVEL check with rootLogger.child({ script: 'indexer', contract: CONTRACT_ID, rpc: RPC_URL })
• All call sites use logger.debug/info/error with structured context fields
• console.error kept only in pre-init CONTRACT_ID guard and main().catch fatal path

scripts/health-check.ts

• Remove stale duplicate imports and local timestamp() / log() helpers
• Use rootLogger.child({ script: 'health-check', contract: CONTRACT_ID, rpc: RPC_URL })
• All call sites use logger.info/error with structured context

scripts/alert-failed-charges.ts

• Fix stale console.error syntax error in sendWebhook; correct misuse of logger.error for info-level events
• Use logger.child({ script: 'alert-failed-charges', db: dbPath }) in main() (no contract/rpc — not applicable for a DB-only script)
• console.error kept only in pre-init WEBHOOK_URL guard

scripts/README.md

• Add JSON log line example to the Logs and metrics section under Keeper
• Add LOG_LEVEL and LOG_FORMAT rows to the keeper environment variable table and the full environment matrix

Acceptance Criteria

• All four scripts use the shared logger for non-fatal logs
• LOG_LEVEL is respected via parseLevel(process.env.LOG_LEVEL) in createLogger
• Required fields (script, contract, rpc where applicable) appear on every log line
• README shows one example JSON log line from keeper

Example log line (keeper, LOG_FORMAT=json)

{"timestamp":"2026-08-30T17:45:00.123Z","level":"INFO","message":"Keeper started in LIVE mode","script":"keeper","contract":"CAAAA...","rpc":"https://soroban-testnet.stellar.org","mode":"live"}

Closes #878

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