Store a target's settings as columns (closes #543)

A target's settings move from the per-type JSON string in
targets.config to columns: url, headers, timeout_seconds,
forward_query, archive_expiry and archive_rotation. A column a type
does not use stays empty. headers holds the header map as JSON text
through GORM's JSON serializer, and archive_expiry holds a
time.Duration, 0 meaning never, so nothing that reads a target parses
it. The forms check each setting once, with the same refusals and
messages, and the edit form is filled from the columns, so the
per-type config structs, their parsers and the "could not be read"
branches are gone. A delivery task carries the six settings in place
of the JSON string.

Model: opus-5-5

internal/handlers/archive_expiry.go

@@ -1,8 +1,14 @@
 package handlers
 
+import (
+	"strconv"
+	"time"
+)
+
 const (
 	// archiveExpiryNever is the archive expiry that keeps archived
-	// events forever. A stored empty expiry means the same.
+	// events forever. It is stored as an ArchiveExpiry of 0, and an
+	// empty expiry on a form means the same.
 	archiveExpiryNever = "never"
 
 	// tmplKeyArchiveExpiryChoices is the template data key for the
@@ -56,3 +62,19 @@ func archiveExpiryOptions(expiry string) []archiveChoice {
 
 	return append([]archiveChoice{own}, options...)
 }
+
+// formatArchiveExpiry writes a stored ArchiveExpiry the way the
+// expiry select spells its values, so the target edit form starts on
+// the stored choice: never for 0, a whole number of hours as "720h",
+// and anything else as Go writes a duration, which the form accepts
+// back unchanged.
+func formatArchiveExpiry(expiry time.Duration) string {
+	switch {
+	case expiry == 0:
+		return archiveExpiryNever
+	case expiry%time.Hour == 0:
+		return strconv.FormatInt(int64(expiry/time.Hour), 10) + "h"
+	default:
+		return expiry.String()
+	}
+}