Add hash field expiration support for HashModel (Redis 7.4+)

[abrookins]

Summary

Add support for Redis 7.4+ hash field expiration to HashModel. This allows setting TTL on individual hash fields rather than the entire key.

Redis Commands (7.4+)

• HEXPIRE / HPEXPIRE - Set TTL on hash fields (seconds/milliseconds)
• HEXPIREAT / HPEXPIREAT - Set expiration at Unix timestamp
• HTTL / HPTTL - Get TTL of hash fields
• HPERSIST - Remove expiration from hash fields

redis-py Support

These commands are available in redis-py >= 5.1.0 (PR #3218, merged June 2024).

Proposed API

Declarative (field-level default TTL)

class Session(HashModel):
    user_id: str
    token: str = Field(expire=3600)  # Default TTL of 1 hour

Runtime methods

session = Session(user_id="123", token="abc")
await session.save()

# Set/update field expiration
await session.expire_field("token", 3600)

# Get remaining TTL
ttl = await session.field_ttl("token")

# Remove expiration
await session.persist_field("token")

Per-save TTL

await session.save(field_expirations={"token": 3600})

Implementation Checklist

• Extend FieldInfo and Field() with expire parameter
• Add instance methods: expire_field(), field_ttl(), persist_field()
• Modify HashModel.save() to apply field expirations
• Add runtime check for redis-py version / Redis server version
• Tests with Redis 7.4+
• Documentation updates