design bounded arbitrary-token Activity discovery

## Problem

#509 must remove the unbounded 31-day all-contract CoinbaSeQL scan introduced in #445 because CDP rejects it after exceeding a 13.97 GiB query-memory limit. Home still needs a bounded way to discover arbitrary received ERC-20 tokens.

## Scope

- Evaluate provider/indexed-table, webhook, or bounded RPC alternatives that are keyed by the verified owner without scanning all Base ERC-20 events.
- Preserve exact contract identity, NFT filtering, reorg behavior, and deterministic pagination.
- Do not make the configured-asset Activity feed depend on this supplemental source.

## Acceptance

- Live provider proof on a wallet with an arbitrary received ERC-20.
- Bounded query/resource behavior.
- Typed partial-source behavior and regression coverage.

Follow-up from #509.

Start by reading #509 and the implementation introduced by #445 to understand the current Activity discovery path and its failure mode. Compare the provider, indexed-table, webhook, and bounded RPC options against exact contract identity, NFT filtering, reorg behavior, and deterministic pagination. Done means live proof, bounded resource use, typed partial-source behavior, and regression coverage without changing the configured-asset feed.

Written by the indexing model from the issue text.