chore: switch database engine to sqlite (fixes #9954) (#9965)

Switch the database from LevelDB to SQLite, for greater stability and
simpler code.

Co-authored-by: Tommy van der Vorst <tommy@pixelspark.nl>
Co-authored-by: bt90 <btom1990@googlemail.com>

internal/itererr/itererr.go

@@ -0,0 +1,83 @@
+// Copyright (C) 2025 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package itererr
+
+import "iter"
+
+// Collect returns a slice of the items from the iterator, plus the error if
+// any.
+func Collect[T any](it iter.Seq[T], errFn func() error) ([]T, error) {
+	var s []T
+	for v := range it {
+		s = append(s, v)
+	}
+	return s, errFn()
+}
+
+// Zip interleaves the iterator value with the error. The iteration ends
+// after a non-nil error.
+func Zip[T any](it iter.Seq[T], errFn func() error) iter.Seq2[T, error] {
+	return func(yield func(T, error) bool) {
+		for v := range it {
+			if !yield(v, nil) {
+				break
+			}
+		}
+		if err := errFn(); err != nil {
+			var zero T
+			yield(zero, err)
+		}
+	}
+}
+
+// Map returns a new iterator by applying the map function, while respecting
+// the error function. Additionally, the map function can return an error if
+// its own.
+func Map[A, B any](i iter.Seq[A], errFn func() error, mapFn func(A) (B, error)) (iter.Seq[B], func() error) {
+	var retErr error
+	return func(yield func(B) bool) {
+			for v := range i {
+				mapped, err := mapFn(v)
+				if err != nil {
+					retErr = err
+					return
+				}
+				if !yield(mapped) {
+					return
+				}
+			}
+		}, func() error {
+			if prevErr := errFn(); prevErr != nil {
+				return prevErr
+			}
+			return retErr
+		}
+}
+
+// Map returns a new iterator by applying the map function, while respecting
+// the error function. Additionally, the map function can return an error if
+// its own.
+func Map2[A, B, C any](i iter.Seq[A], errFn func() error, mapFn func(A) (B, C, error)) (iter.Seq2[B, C], func() error) {
+	var retErr error
+	return func(yield func(B, C) bool) {
+			for v := range i {
+				ma, mb, err := mapFn(v)
+				if err != nil {
+					retErr = err
+					return
+				}
+				if !yield(ma, mb) {
+					return
+				}
+			}
+		}, func() error {
+			if prevErr := errFn(); prevErr != nil {
+				return prevErr
+			}
+			return retErr
+		}
+}