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

A database target's rotation setting (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 and both target forms, and shown in the target list. Renames move every one of a target's files and move them all back if one fails. The sweep prunes one file at a time under the target's lock and deletes a rotated file it leaves empty. Download opens one file at a time, oldest period first, finding each again under the target's current name. The target list names the current file and totals all of them.

Model: opus-5-5

internal/delivery/target_config_view.go

@@ -192,8 +192,9 @@ func maxRetriesField(t *database.Target) ConfigField {
 
 // databaseConfigFields describes an archive target by its
 // expiry in plain units, such as "30 days", or "never" when
-// the archive is kept forever. An expiry that is set but not
-// a valid duration is reported as unavailable rather than
+// the archive is kept forever, and by its rotation. An expiry
+// that is set but not a valid duration, or a rotation that is
+// not one of the four, is reported as unavailable rather than
 // echoed back.
 func databaseConfigFields(configJSON string) []ConfigField {
 	expiry, err := parseArchiveExpiry(configJSON)
@@ -201,6 +202,11 @@ func databaseConfigFields(configJSON string) []ConfigField {
 		return unavailableConfigFields()
 	}
 
+	rotation, err := parseArchiveRotation(configJSON)
+	if err != nil {
+		return unavailableConfigFields()
+	}
+
 	value := archiveExpiryNever
 	if expiry > 0 {
 		value = plainDuration(expiry)
@@ -209,6 +215,9 @@ func databaseConfigFields(configJSON string) []ConfigField {
 	return []ConfigField{{
 		Label: "Archive Expiry",
 		Value: value,
+	}, {
+		Label: "Archive Rotation",
+		Value: rotation,
 	}}
 }