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. 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; tests check that each copy
into a task, and back out of one, keeps all six.

Model: opus-5-5

internal/delivery/target_redact.go

@@ -18,7 +18,7 @@ const RedactionMarker = "(redacted)"
 // line behind the delivery path's own URL masking.
 //
 // It removes byte-identical echoes of strings taken from the
-// target's stored configuration, and nothing else. Anything
+// target's URL and headers, and nothing else. Anything
 // the remote re-encodes survives: JSON "\/" escaping (what
 // PHP's json_encode emits by default), percent-encoding, HTML
 // entities, and an echo of only part of a path. It cannot
@@ -115,7 +115,7 @@ func (r Redactor) secretPrefixSuffix(s string) int {
 }
 
 // targetSecrets returns the credential-bearing strings a
-// target's configuration carries.
+// target's settings carry.
 //
 // The destination URL contributes. Its path, query and
 // userinfo are the credential for both target types that have
@@ -129,36 +129,15 @@ func (r Redactor) secretPrefixSuffix(s string) int {
 // picks out. That is the same class-based rule applied to
 // URLs: an echoed Accept or User-Agent still renders, an
 // echoed Authorization does not.
+//
+// A setting a type does not use is empty and contributes
+// nothing, so a database or log target has nothing to redact.
 func targetSecrets(t *database.Target) []string {
 	if t == nil {
 		return nil
 	}
 
-	switch t.Type {
-	case database.TargetTypeSlack:
-		cfg, err := parseSlackConfig(t.Config)
-		if err != nil {
-			return nil
-		}
-
-		return urlSecrets(cfg.WebhookURL)
-	case database.TargetTypeHTTP:
-		cfg, err := parseHTTPConfig(t.Config)
-		if err != nil {
-			return nil
-		}
-
-		return append(
-			urlSecrets(cfg.URL),
-			headerSecrets(cfg.Headers)...,
-		)
-	case database.TargetTypeDatabase, database.TargetTypeLog:
-		// Neither has a destination URL, so neither has
-		// anything to redact.
-		return nil
-	default:
-		return nil
-	}
+	return append(urlSecrets(t.URL), headerSecrets(t.Headers)...)
 }
 
 // urlSecrets returns the substrings of a destination URL that