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/users.sql

@@ -37,7 +37,7 @@ SELECT count(*) FROM users;
 -- name: SetSubsonicPassword :exec
 -- Stores (or clears with NULL) the per-user Subsonic legacy credential used
 -- for t/s and p auth on /rest/*. Must be plaintext; see migration 0003. Only
--- ever a server-generated value, never the login password (#5026).
+-- ever a server-generated value, never the login password.
 UPDATE users SET subsonic_password = $2 WHERE id = $1;
 
 -- name: GetUserByID :one