feat: RDB-like persistence (SAVE, BGSAVE, auto-load)

[zulerne]

Implement snapshot-based persistence similar to Redis RDB.

Commands

• `SAVE` — synchronous snapshot to disk
• `BGSAVE` — background snapshot (goroutine)

Behavior

• On startup, load the last snapshot if it exists
• Configurable save interval (e.g. every N seconds if M keys changed)
• Binary or GOB encoding for the data store
• Write to temp file, then atomic rename

Implementation

• New `internal/store/persistence.go`
• Snapshot format: encode `map[string]*Item` with GOB or custom binary format
• Config: snapshot path, auto-save interval
• SAVE/BGSAVE handlers in `internal/commands/server.go`