feat(ntx-builder): materialize note eligibility in the database

[SantiagoPittella]

Summary

Adds notes.next_eligible_block: the first block at which a note may be attempted, being the later of its execution-hint window and its post-failure backoff. Every write path maintains it, and the backoff and hint arithmetic moves into db::eligibility so the read and write paths share one definition.

Nothing reads the column yet.

A sponsorship arriving for a pending feature note now clears that note's backoff, since the note may have failed for an uncovered fee and the sponsorship is new information. Existing rows default to 0 (eligible now).

Changelog

[[entry]]
scope       = "ntx-builder"
impact      = "added"
description = "Added the `notes.next_eligible_block` column."