Add /api/v1/address/{addr}/txs (transaction history)

[Bortlesboat]

Summary

Add transaction history for addresses. Currently we have balance + UTXOs via `scantxoutset`, but no tx history.

Challenge

Bitcoin Core doesn't index by address natively. Options:

1. Electrs/Fulcrum integration — external indexer, most complete
2. scanblocks RPC (Bitcoin Core 26+) — scan blocks for address matches, slow but no deps
3. listtransactions — only for wallet addresses

Proposed approach

Start with option 2 (scanblocks) for a dependency-free version, with optional Electrs backend for power users.

Acceptance criteria

• `GET /api/v1/address/{addr}/txs` returns paginated tx list
• Works without Electrs (basic mode)
• Optional Electrs integration for full history
• Tests

[Bortlesboat]

Grant-roadmap context: this existing address transaction-history issue is the public starting point for the compact address-indexer work described in the HRF proposal. The near-term goal is a dependency-light path for private address history lookups, with optional heavier backends only where they genuinely improve self-hosted operation.