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/0028_discovery_mix_variants.up.sql

@@ -1,11 +1,9 @@
--- #411 R3 bugfix: the five discovery mixes (deep_cuts, rediscover,
--- new_for_you, on_this_day, first_listens) were added as producers +
--- registry entries but their system_variant values were never added
--- to the two playlists CHECK constraints. Result: insertSystemPlaylist
--- for any new mix fails with playlists_kind_variant_consistent
--- (SQLSTATE 23514), which — because BuildSystemPlaylists is one
--- all-or-nothing transaction — aborts the WHOLE build, so For-You /
--- Discover refresh 500s and the lazy daily build fails too.
+-- Before: the five discovery mixes (deep_cuts, rediscover, new_for_you,
+-- on_this_day, first_listens) had producers and registry entries but no
+-- system_variant values in the two playlists CHECK constraints, so inserting
+-- any of them failed playlists_kind_variant_consistent (SQLSTATE 23514) —
+-- and because BuildSystemPlaylists is one all-or-nothing transaction, that
+-- aborts the WHOLE build. After: the constraints accept them.
 --
 -- All five are seedless (like for_you/discover): seed_artist_id IS
 -- NULL. Drop + re-add both constraints with the expanded set, same