Add persistent storage using BoltDB or BadgerDB

[devduart]

Summary

Replace in-memory blockchain with persistent key-value storage to preserve data between sessions.

Tasks

• Choose and integrate BoltDB or BadgerDB
• Store each block with its hash as key
• On startup, load the blockchain from the database
• Implement block existence check and block-by-hash retrieval
• Refactor Blockchain struct to include DB reference if needed

Options

• https://github.com/etcd-io/bbolt (BoltDB)
• https://github.com/dgraph-io/badger (BadgerDB)

Benefit

Enables durability and scalability for future node syncing and inspection tools.