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/0068_leveled_stream_cache.up.sql

@@ -1,11 +1,10 @@
 -- 0068_leveled_stream_cache.up.sql — the size cap of the leveled-stream render
--- cache (Scribe milestone #464, #5001).
+-- cache.
 --
 -- Sonos and other UPnP renderers fetch a track's URL themselves, so the phone
 -- cannot level what they play. The server renders a gain-applied FLAC for them
 -- instead and keeps it on disk, oldest-used first out once the cache is over
--- this size. An operator setting (rule 25), on the loudness settings row it
--- belongs with.
+-- this size. An admin setting, on the loudness settings row it belongs with.
 ALTER TABLE loudness_settings
     ADD COLUMN leveled_cache_mb integer NOT NULL DEFAULT 2048,
     ADD CONSTRAINT loudness_settings_leveled_cache_range