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/0064_clear_subsonic_password.up.sql

@@ -1,9 +1,9 @@
--- `minstrel admin reset-password` used to copy the new login password into
+-- Before: `minstrel admin reset-password` copied the new login password into
 -- subsonic_password, so every account recovered through the CLI had its login
 -- password stored in plain text, and a later password change in Settings left
--- that copy behind (M462 #5026). The CLI no longer writes this column; a
--- Subsonic password is now generated separately in Settings and is never the
--- login password. Clearing every value here removes the copies already made.
+-- that copy behind. After: the CLI does not write this column; a Subsonic
+-- password is generated separately in Settings and is never the login
+-- password. Clearing every value here removes the copies already made.
 --
 -- Accounts whose Subsonic client signs in with t/s stop working until the user
 -- generates a Subsonic password (or switches the client to an API key).