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/0061_fingerprint_settings.up.sql

@@ -1,20 +1,18 @@
--- 0061_fingerprint_settings.up.sql — fingerprinting's knobs, in admin Settings
--- (Scribe #3913, milestone #400). Rule 25: anything an operator might tune is a
--- database row, changed without a restart. Singleton in the style of
--- reacquisition_settings (0056).
+-- 0061_fingerprint_settings.up.sql — fingerprinting's knobs, in admin
+-- Settings: anything an admin might tune is a database row, changed without a
+-- restart. Singleton in the style of reacquisition_settings (0056).
 CREATE TABLE fingerprint_settings (
     id                          boolean PRIMARY KEY DEFAULT true,
 
-    -- Fingerprinting new files, the backfill, and the duplicate sweep. Off stops
-    -- the decode work entirely — the reason to turn it off is a slow NAS, and
-    -- that is the operator's call. On by default: a library that cannot tell its
-    -- duplicates apart is what milestone #400 exists to end.
+    -- Fingerprinting new files, the backfill, and the duplicate sweep. Off
+    -- stops the decode work entirely — the reason to turn it off is a slow NAS.
+    -- On by default: without it the library cannot tell its duplicates apart.
     enabled                     boolean NOT NULL DEFAULT true,
 
     -- Seconds of audio fpcalc fingerprints. Chromaprints taken at different
     -- lengths cannot be compared, which is why track_fingerprints records the
-    -- length each row was taken at (below): change this and every chromaprint is
-    -- re-derived, and until then only rows at the new length are compared.
+    -- length each row was taken at (below): change this and every chromaprint
+    -- is re-derived, and until then only rows at the new length are compared.
     chromaprint_length_sec      integer NOT NULL DEFAULT 120,
 
     -- The most disagreement two aligned fingerprints may show and still be