feat(validator): paginated admin transaction listing

[sergerad]

Summary

Part 2 of a 4-PR stack formalizing the validator admin API (#2455).

The admin listing returned every stored record in one response, which OOMs/breaks on any non-trivial history. Replace it with a paginated listing of committed transactions in committed order, and version the admin paths under /admin/v1/.

• GET /admin/v1/transactions?block_from=&tx_index_from=&block_to=&limit=&include_records=: rows ordered by (block_num, block_tx_index) — the only order with any bearing on the chain. In-flight / never-committed rows are not listed (they remain reachable by id).
• Pagination is strict keyset paging: (block_from, tx_index_from) is the cursor, and limit is honored exactly. Responses follow the PaginationInfo convention of the node's sync_* RPCs, carrying { chain_tip, block_num, block_tx_index }; the next page is the same request resumed one position past the last row (block_from = block_num, tx_index_from = block_tx_index + 1). No opaque cursor.
• Items are metadata-only (tx id, position, key epoch, setup context id) unless include_records=true, which loads each row's full sealed record by id under a lower limit cap — the full-record-per-row response was most of the memory problem.
• GET /admin/v1/transactions/{tx_id} returns one full record by id, including for uncommitted transactions.

The listing query costs one indexed seek per page: the row-value comparison (block_num, block_tx_index) >= (?, ?) seeks the link table's primary key directly (verified under EXPLAIN QUERY PLAN: no scan, no sort step).

Stack: #2531 → #2532 → #2533 → #2517

Changelog

[[entry]]
scope       = "validator"
impact      = "changed"
description = "The validator admin transaction listing is paginated (strict keyset pages in committed order, PaginationInfo-style cursor) and returns metadata-only items unless full records are requested; admin paths moved under /admin/v1/."

[[entry]]
scope       = "validator"
impact      = "added"
description = "GET /admin/v1/transactions/{tx_id} returns one validated transaction's full sealed record by id."