feat: add cron engine with TTL triggers, Redis CLI, and 6-field cron support

[nirmash]

Summary

Adds three major features to PulseBoard:

Cron Engine (TTL Triggers)

A cron job scheduler that uses Valkey key expiration as the trigger mechanism — no external scheduler needed.

• How it works: Cron expressions are parsed into seconds-until-next-fire, set as a TTL on a Redis trigger key. When the key expires, a keyspace notification fires the job and recreates the key for the next run.
• Supports both HTTP (PUT) and shell script execution
• Job definitions persist in Redis hashes — restored on server restart
• Dedicated ioredis subscriber connection for __keyevent@0__:expired events
• Pause/resume, delete, and execution log per job

6-Field Cron with Seconds

Extended cron parser supports both formats:

• 5 fields (standard): min hr dom mon dow
• 6 fields (with seconds): sec min hr dom mon dow

Enables sub-minute scheduling (e.g., * * * * * * = every second).

Interactive Redis CLI

Terminal-style Redis client in the browser:

• Execute any safe Redis command
• Arrow-key command history
• CLEAR to reset output
• Dangerous commands blocked server-side

Other Changes

• Cache mode switched to managed in embr.yaml
• Added scripts/log-datetime.sh sample script for cron testing
• Comprehensive README with features, cache modes, and configuration docs
• Consistent panel spacing in the UI

New Files

File	Purpose
server/cron/parser.js	5/6-field cron expression → TTL converter
server/cron/engine.js	Keyspace notification listener + job executor
server/routes/cron.js	Cron job CRUD API
server/routes/redisCli.js	Safe Redis command execution API
client/src/CronJobs.jsx	Cron engine UI
client/src/RedisCli.jsx	Redis CLI UI
scripts/log-datetime.sh	Sample cron script
README.md	Full documentation

Tested

• Deployed to Embr with managed cache mode
• Cron job created, fired, logged, and rescheduled successfully
• Script execution verified via embr shell (/tmp/cron-log.txt populated)
• Redis CLI tested with KEYS, TTL, and other commands