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/0051_discover_tuning.up.sql

@@ -1,14 +1,14 @@
 -- 0051_discover_tuning.up.sql — tunable knobs for the Discover request
--- surface (#2377, milestone #268 slice 6).
+-- surface.
 --
 -- A FOURTH tuning scope alongside radio / daily_mix / taste. Its own scope
 -- rather than extra columns on taste_tuning, for a reason that matters:
 -- snooze_days lives here, and a snooze must never be read as taste signal
--- (#2374). Filing it under 'taste' would put it one careless join away from
--- exactly the leak that design forbids.
+-- (0049). Filing it under 'taste' would put it one careless join away from
+-- exactly that leak.
 --
--- Per rule #25 these are DB-backed and editable in the admin UI with no
--- restart — the shipped values below are defaults, not settings.
+-- These are DB-backed and editable in the admin UI with no restart — the
+-- shipped values below are defaults, not settings.
 CREATE TABLE discover_tuning (
     singleton          boolean PRIMARY KEY DEFAULT true
         CONSTRAINT discover_tuning_singleton_check CHECK (singleton),
@@ -17,10 +17,10 @@ CREATE TABLE discover_tuning (
     -- [0,1]. So 0 disables the feature outright and leaves pure similarity
     -- ranking, 1.0 lets a perfectly-matching candidate double its score, and
     -- a candidate with no cached tags is unchanged rather than penalised
-    -- (rule #131 — tag coverage is permanently partial, see #2376).
+    -- (tag coverage is permanently partial, so a missing tag is no signal).
     tag_overlap_weight double precision NOT NULL,
-    -- Default snooze duration in days. Was a Go constant in
-    -- internal/api/suggestions.go; moved here per rule #25.
+    -- Default snooze duration in days. Before this a Go constant in
+    -- internal/api/suggestions.go; now an admin setting.
     snooze_days        double precision NOT NULL,
     updated_at         timestamptz NOT NULL DEFAULT now()
 );