refactor(indexer): declarative event handler registry

[satyakwok]

Summary

Tier 3 refactor — replaces the hardcoded ERC-20 / ERC-721 / ERC-1155 dispatch in `sync.ts` with a registry where each event type owns its own decoder file. Adding a new event (DEX swap, NFT mint, custom protocol log) is now a one-file addition under `apps/indexer/src/handlers/` plus one line in `handlers/index.ts` — no `sync.ts` edit, no growing `if/else` in the hot loop.

Layout

```
apps/indexer/src/handlers/
├── registry.ts EventHandler interface + register/dispatch + topicToAddress
├── erc20.ts ERC-20 Transfer (3 topics)
├── erc721.ts ERC-721 Transfer (4 topics, shares topic0 with ERC-20)
├── erc1155.ts TransferSingle (decoded) + TransferBatch (deferred null-return)
└── index.ts Side-effect import barrel
```

How shared topic0 works

The registry keys handlers by `topic0`; multiple handlers can share a `topic0` (the dispatcher walks them in registration order, keeps the first non-null result). That's how the ERC-20 vs ERC-721 split works: both register against `keccak("Transfer(address,address,uint256)")` but each null-returns when the topic count doesn't match its arity.

Behaviour parity

Identical to pre-refactor — same output rows for the same input logs. The refactor is type-safe end-to-end because `EventHandler.decode` returns the Drizzle-inferred `TransferRow` type.

Adding a new handler

```ts
// apps/indexer/src/handlers/uniswap-v2-swap.ts
import { register, type EventHandler } from "./registry.js";

const handler: EventHandler = {
topic0: "0xd78ad95fa46c994b6551d0da85fc275fe613ce37657fb8d5e3d130840159d822",
decode: ({ log, contract, txHash }) => { /* … */ },
};
register(handler);
```

Then `import "./uniswap-v2-swap.js";` in `handlers/index.ts`. Zero `sync.ts` change.

Test plan

• `pnpm turbo build` passes
• `pnpm db:migrate` no-op on staging
• Index a known-good block with mixed ERC-20 + ERC-721 + ERC-1155 transfers; verify output rows match what the pre-refactor code produced (compare `SELECT * FROM token_transfers WHERE block_height = X`)