chore(migrations): renumber muted_recipients 000061 to 000076

Rebasing #297 onto feat/multicloud-web-frontend surfaced a migration-number
collision: the base sequence already extends past 000061 (tops at 000073),
and 000074/000075 are taken by in-flight PRs #1011 and #1094. Move the
per-recipient mute migration to the next free slot, 000076, for both the
.up.sql and .down.sql, so the merge-ref check-migration-conflicts hook stays
green. Update the matching "migration 000061" reference in the
config.StoreInterface doc comment.

Author: cristim

internal/config/interfaces.go

@@ -304,7 +304,7 @@ type StoreInterface interface {
 	// supported — fn must not call WithTx recursively.
 	WithTx(ctx context.Context, fn func(tx pgx.Tx) error) error
 
-	// Notification mutes (issue #297 / migration 000061).
+	// Notification mutes (issue #297 / migration 000076).
 	// UpsertNotificationMute inserts or updates a mute row for (email, scope).
 	// Idempotent: calling it again for an already-muted address is a no-op on
 	// muted_at but does replace unmute_token if the token changes.