feat: add Hash data type (HSET, HGET, HGETALL, HDEL, HEXISTS, HLEN)

[zulerne]

Hash is the most commonly used Redis type after strings and lists. Needed for the messenger project (user sessions, metadata storage).

Commands

• HSET <key> <field> <value> [field value ...]
• HGET <key> <field>
• HGETALL <key>
• HDEL <key> <field> [field ...]
• HEXISTS <key> <field>
• HLEN <key>

Implementation

• New TypeHash DataType = "hash" in store
• Store value as map[string]string
• New internal/commands/hash.go for handlers
• Tests in internal/store/store_test.go