docs: query comments say what each query selects and why, not how it came to be (#6277)

Rewrites the internal/db/queries comments to rule 32 and regenerates
dbq, whose doc comments and embedded SQL carry them. The bugs behind the
tiered mixes, the md5 order seed, the score bands and the round-robin
arms are restated as the constraint each one enforces; counts from one
library and stale cross-references are gone.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/db/queries/library_changes.sql

@@ -3,11 +3,10 @@ INSERT INTO library_changes (entity_type, entity_id, op)
 VALUES ($1, $2, $3);
 
 -- name: InsertLibraryChanges :exec
--- Batch form, for a mutation that touches many rows at once (#2704: the scan's
--- reconcile pass can mark up to a quarter of a library missing in one go).
--- Every caller before this one changed a single entity, so per-row inserts
--- were the right shape; a loop here would be thousands of round-trips inside
--- a scan that is already the slow path.
+-- Batch form, for a mutation that touches many rows at once (the scan's
+-- reconcile pass can mark up to a quarter of a library missing in one go). A
+-- loop of single inserts would be thousands of round-trips inside a scan that
+-- is already the slow path.
 INSERT INTO library_changes (entity_type, entity_id, op)
 SELECT sqlc.arg(entity_type)::text,
        unnest(sqlc.arg(entity_ids)::text[]),