docs: migration headers keep the before and after, without ids, quotes or one library's numbers (#6277)

Applies rule 32's migration exception: each header still says what the
schema was, what changes and what it prevents, but task, milestone,
decision and rule ids are replaced by the practice they stood for, the
operator quotes by the decision, single-install counts are dropped, and
pointers to a deleted package and an external design doc are gone.
golang-migrate keeps no checksum, so editing applied migrations'
comments is safe.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/db/migrations/0018_artist_art_and_provider_settings.up.sql

@@ -1,6 +1,5 @@
--- M7 cover-sources: schema for additional cover-art providers (TheAudioDB
--- as the v1 implementation), artist art (thumb + fanart), and the
--- recheck-on-source-change trigger.
+-- Schema for additional cover-art providers (TheAudioDB first), artist art
+-- (thumb + fanart), and the recheck-on-source-change trigger.
 
 -- (0) Extend the existing albums.cover_art_source CHECK to accept the
 -- new 'theaudiodb' value. The constraint was created in migration 0016