Design and migrate time-series schema for wallet balance history

[christabel888]

Context: Schema work backing the Wallet Dashboard's balance-history endpoint (#26).

Where:

• New migrations/033_create_wallet_balance_history.sql

What to do:

• Table for periodic per-wallet balance snapshots (address, asset, balance, snapshot_time).

Suggested approach:

1. Table: address, asset, balance, snapshot_time -- straightforward, but think about the primary key/uniqueness constraint (one row per address+asset+snapshot_time) up front.
2. This directly backs Add /api/v1/wallets/{address}/balance-history endpoint #26 -- coordinate on exact column names/types so Add /api/v1/wallets/{address}/balance-history endpoint #26 doesn't need a follow-up migration.

Watch out for:

• This table could grow very large very fast if snapshotting isn't scoped to "wallets someone has actually queried" (per Add /api/v1/wallets/{address}/balance-history endpoint #26's note) -- don't snapshot the whole network by default.

Definition of done:

• Migration applies cleanly
• Add /api/v1/wallets/{address}/balance-history endpoint #26 can query it directly