merge trails function added (#627)

* merge trails function added

* fix conflict resolve issues

* avoid incorrect authorship when merging trail comments

* require edit access for all selected trails before merge

* fix likes checkbox binding in trail merge modal

* improve error handling

* require editable target and deletable sources for merge

* avoid duplicate trail fetch during merge

* prevent duplicate likes when merging trails

* check photo download responses during trail merge

* add merge completion toast notifications

* cleanup

* small fixes

* move merge code to backend, option to merge single trail selection with similar one, automatic merge for integrations add as option, maintenance page to find and merge similar trails added

* fix build error

* fix

* docu, beautifying, refactoring

---------

Co-authored-by: Flomp <Flomp@users.noreply.github.com>

db/util/distance.go

@@ -0,0 +1,18 @@
+package util
+
+import "math"
+
+func HaversineDistanceMeters(lat1 float64, lon1 float64, lat2 float64, lon2 float64) float64 {
+	const earthRadius = 6371000.0
+
+	lat1Rad := lat1 * math.Pi / 180
+	lat2Rad := lat2 * math.Pi / 180
+	dLat := (lat2 - lat1) * math.Pi / 180
+	dLon := (lon2 - lon1) * math.Pi / 180
+
+	sinLat := math.Sin(dLat / 2)
+	sinLon := math.Sin(dLon / 2)
+
+	h := sinLat*sinLat + math.Cos(lat1Rad)*math.Cos(lat2Rad)*sinLon*sinLon
+	return 2 * earthRadius * math.Atan2(math.Sqrt(h), math.Sqrt(1-h))
+}