Add structured sample-pack license terms and delivery

Commit notes

Introduce versioned, BeatStars-style license templates (royalty-free standard + exclusive) as a backend source of truth, let sellers pick a license per pack, disclose the buyer's rights at the point of sale, snapshot the granted license immutably onto each purchase, and deliver a personalized LICENSE.txt inside every download zip. Adds a public /legal/licensing page.

Migration 0028 adds sample_packs.license_type and the purchases license snapshot columns (hand-written + journal entry, matching the 0024-0027 convention; snapshots intentionally stop at 0023 in this repo).

Co-Authored-By: Claude Opus 4.8 <[email redacted]>