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=&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 follows the PaginationInfo convention of the node's sync_* RPCs: responses carry { chain_tip, block_num }, and the next page is the same request with block_from = block_num + 1. No opaque cursor.
• Pages hold whole blocks: limit is a row-count target and the block that crosses it is served in full, so a page always ends on a block boundary.
• Items are metadata-only (tx id, position, key epoch, setup context id) unless include_records=true, which carries the full sealed record 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 (O(log n), O(n) per sweep). The SQL comment documents why the resume point must be folded into the range's lower bound rather than passed as a second predicate: the two forms plan identically under EXPLAIN QUERY PLAN but the latter measured two orders of magnitude slower.

Stack: #2531 → #2532 → #2533 → #2517

Changelog

[[entry]]
scope       = "validator"
impact      = "changed"
description = "The validator admin transaction listing is paginated (whole-block 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."