Add configurable TTL for Redis task data

[hoi]

Summary

• Add configurable TTL (Time-To-Live) for Redis task data to prevent unbounded memory growth
• Task data automatically expires after 1 hour by default (configurable)
• Supports runtime override via REDIS_TASK_TTL environment variable

Changes

Configuration

• Added task_ttl_seconds to config.yml under redis section (default: 3600)
• Added REDIS_TASK_TTL environment variable support for runtime override
• Setting TTL to 0 disables automatic expiration

Implementation

• Added get_redis_task_ttl() helper function in utils.py
• Added hset_with_ttl() async helper in api.py for consistent TTL application
• Updated all 8 Redis task storage calls to use TTL

Documentation

• Updated config.yml with TTL documentation
• Updated README.md with TTL configuration details

Test plan

• Unit tests for get_redis_task_ttl() function (4 tests)
• Unit tests for TTL environment variable override (3 tests)
• Unit tests for hset_with_ttl() behavior (4 tests)
• Unit tests for config documentation (2 tests)
• All 13 tests pass