Rotate a database target's archive monthly, daily or hourly (closes #379)

A database target's rotation (none, monthly, daily or hourly) puts the
UTC period of each event's receive time in its archive file name, so
each file holds exactly its period's events. It is on the new webhook
page, the add target form and the target edit form, and shown in the
target list.

Renames move every one of a target's files and move them back if one
fails. The sweep prunes every file, one at a time under the target's
lock, and deletes a rotated file it leaves empty. Download lists the
files, then opens one at a time, oldest first, finding each again under
the target's current names, and gives each row its period. The target
list names the current file and totals the size of all of them.

Model: opus-5-5

internal/handlers/archive_expiry.go

@@ -10,10 +10,10 @@ const (
 	tmplKeyArchiveExpiryChoices = "ArchiveExpiryChoices"
 )
 
-// archiveExpiryChoice is one entry of a database target's archive
-// expiry select: the expiry stored, the label shown, and whether the
-// select starts on it.
-type archiveExpiryChoice struct {
+// archiveChoice is one entry of a database target's archive expiry
+// or archive rotation select: the value stored, the label shown, and
+// whether the select starts on it.
+type archiveChoice struct {
 	Value    string
 	Label    string
 	Selected bool
@@ -21,8 +21,8 @@ type archiveExpiryChoice struct {
 
 // archiveExpiryChoices lists the archive expiries offered by the new
 // webhook page, the add target form and the target edit form.
-func archiveExpiryChoices() []archiveExpiryChoice {
-	return []archiveExpiryChoice{
+func archiveExpiryChoices() []archiveChoice {
+	return []archiveChoice{
 		{Value: archiveExpiryNever, Label: archiveExpiryNever},
 		{Value: "1h", Label: "1h"},
 		{Value: "12h", Label: "12h"},
@@ -37,7 +37,7 @@ func archiveExpiryChoices() []archiveExpiryChoice {
 // empty expiry selects never. An expiry that is not one of the
 // choices comes first as its own selected entry, so saving the form
 // unchanged keeps it.
-func archiveExpiryOptions(expiry string) []archiveExpiryChoice {
+func archiveExpiryOptions(expiry string) []archiveChoice {
 	if expiry == "" {
 		expiry = archiveExpiryNever
 	}
@@ -52,7 +52,7 @@ func archiveExpiryOptions(expiry string) []archiveExpiryChoice {
 		}
 	}
 
-	own := archiveExpiryChoice{Value: expiry, Label: expiry, Selected: true}
+	own := archiveChoice{Value: expiry, Label: expiry, Selected: true}
 
-	return append([]archiveExpiryChoice{own}, options...)
+	return append([]archiveChoice{own}, options...)
 }