feat(minibf): add /accounts/{stake_address}/addresses/assets endpoint

[slowbackspace]

Summary

Closes #1067 by adding the Blockfrost-compatible GET /accounts/{stake_address}/addresses/assets endpoint.

• Aggregates native assets across the account’s current UTxOs, excluding lovelace
• Supports count, page, and order
• Returns 404 for unknown accounts and existing MiniBF errors for invalid requests and storage failures
• Shares one query facade across block-metadata lookups to respect the blocking-task limit
• Updates MiniBF documentation

Ordering semantics

Because the directions use different keys, desc is not necessarily the reverse of asc.

• asc: each asset’s oldest current UTxO
• desc: each asset’s newest current UTxO
• ties: asset unit ascending

This reproduces the reference implementation (blockfrost-backend-ryo, accounts_stake_address_addresses_assets.sql: MIN(txo.id) ASC / MAX(txo.id) DESC, tie-break (policy, name) ASC), with the tuple (slot, tx_index, output_index) standing in for db-sync's monotone txo.id. The route test accounts_by_stake_addresses_assets_uses_directional_ordering pins the exact asc and desc sequences, including the non-reversal and the tie-break cases.

Testing

• Route-level coverage: aggregation across UTxOs, direction-dependent ordering with position ties and unit tie-breaking (custom fixture state injected via the new TestApp::new_with_cfg_and_setup), pagination including past-the-end pages, invalid stake addresses, unknown accounts, storage failures
• Verified against live Blockfrost preview: the full 116-unit asset list of the blockfrost-tests fixture account is byte-identical in both asc and desc, as are pagination windows and the 400/404 error responses.

Known deviations

• Collateral-return UTxOs of failed txs are included. The reference blockfrost-backend-ryo query predates collateral-return outputs so Blockfrost misses assets held via unspent collateral returns. Dolos aggregates from the ledger-true UTxO set, so these appear naturally. Practically unobservable outside that rare case; same ledger-truth stance as fix(minibf): return affected addresses from /blocks/{hash_or_number}/addresses #1124
• Invalid stake addresses return a bare 400 (no JSON error body) — pre-existing behavior shared by all account endpoints, deliberately left untouched here

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• Commit unit tests in branch feat/minibf-account-addresses-assets

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