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/delivery/target_config_edit.go

@@ -41,6 +41,8 @@ type TargetConfigForm struct {
 	Timeout string
 	// Expiry is the database (archive) target's row expiry.
 	Expiry string
+	// Rotation is the database (archive) target's rotation.
+	Rotation string
 }
 
 // NewTargetConfigForm parses a target's stored configuration into
@@ -85,11 +87,13 @@ func NewTargetConfigForm(
 	}
 }
 
-// databaseConfigForm parses an archive target's optional expiry.
-// An absent, empty or never expiry yields an empty expiry, on which
-// the edit form starts at never; saving it unchanged stores never,
-// which means the same as an empty expiry. An expiry that is set
-// but not a valid duration is an error, not a blank field.
+// databaseConfigForm parses an archive target's optional expiry and
+// rotation. An absent, empty or never expiry yields an empty expiry,
+// on which the edit form starts at never; saving it unchanged stores
+// never, which means the same as an empty expiry. An absent rotation
+// is empty too, and the form starts at none. An expiry that is set
+// but not a valid duration, or a rotation that is not one of the
+// four, is an error, not a blank field.
 func databaseConfigForm(
 	configJSON string,
 ) (TargetConfigForm, error) {
@@ -106,8 +110,15 @@ func databaseConfigForm(
 		)
 	}
 
+	err = ValidateArchiveRotation(cfg.Rotation)
+	if err != nil {
+		return TargetConfigForm{}, err
+	}
+
+	form := TargetConfigForm{Rotation: cfg.Rotation}
+
 	if cfg.Expiry == "" || cfg.Expiry == archiveExpiryNever {
-		return TargetConfigForm{}, nil
+		return form, nil
 	}
 
 	err = ValidateArchiveExpiry(cfg.Expiry)
@@ -115,5 +126,7 @@ func databaseConfigForm(
 		return TargetConfigForm{}, err
 	}
 
-	return TargetConfigForm{Expiry: cfg.Expiry}, nil
+	form.Expiry = cfg.Expiry
+
+	return form, nil
 }