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/0074_notification_email.up.sql

@@ -1,11 +1,11 @@
--- M489 #5346: notifications by email, grouped. Nothing is emailed per event.
+-- Notifications by email, grouped. Nothing is emailed per event.
 --
 -- New music (request_completed) goes out as at most one summary a day, at a set
 -- hour in each user's own timezone. Everything else is batched: a batch opens
 -- at the first un-emailed item and one email goes out a window later, holding
 -- whatever accumulated. internal/notifications/digest.go is the one sender.
 
--- The two knobs, in admin Settings (rule 25). Singleton in the style of
+-- The two knobs, in admin Settings. Singleton in the style of
 -- fingerprint_settings (0061).
 CREATE TABLE notification_email_settings (
     id                    boolean PRIMARY KEY DEFAULT true,