chore(sqlite): stamp files with application_id

No practical effect, just a tiny bit of fun to stamp the database files
with an application ID that identifies them.

Signed-off-by: Jakob Borg <jakob@kastelo.net>

internal/db/sqlite/basedb.go

@@ -25,7 +25,11 @@ import (
 	"github.com/syncthing/syncthing/lib/protocol"
 )
 
-const currentSchemaVersion = 4
+const (
+	currentSchemaVersion = 4
+	applicationIDMain    = 0x53546d6e // "STmn", Syncthing main database
+	applicationIDFolder  = 0x53546664 // "STfd", Syncthing folder database
+)
 
 //go:embed sql/**
 var embedded embed.FS