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/delivery/terminal_state_test.go

@@ -45,7 +45,7 @@ func tSeedDeletedTarget(
 
 	iCreateTarget(
 		t, s.MainDB, targetID, s.WebhookID, name,
-		database.TargetTypeHTTP, iHTTPConfig(url), 5,
+		database.TargetTypeHTTP, url, 5,
 	)
 
 	event := iSeedEvent(
@@ -122,9 +122,9 @@ func TestProcessDelivery_UnknownTargetType_RecordsWhy(
 	)
 
 	target := database.Target{
-		Name:   "mystery",
-		Type:   tUnknownType,
-		Config: iHTTPConfig("http://example.com/hook"),
+		Name: "mystery",
+		Type: tUnknownType,
+		URL:  "http://example.com/hook",
 	}
 	target.ID = targetID
 
@@ -140,7 +140,7 @@ func TestProcessDelivery_UnknownTargetType_RecordsWhy(
 	body := event.Body
 	task := iTask(
 		seeded, event, s.WebhookID, targetID, "mystery",
-		target.Config, 0, 1, &body,
+		target.URL, 0, 1, &body,
 	)
 	task.TargetType = tUnknownType
 
@@ -345,11 +345,10 @@ func tRetryChainSetup(
 	iCreateWebhook(t, s.MainDB, s.WebhookID, name)
 
 	targetID := uuid.New().String()
-	cfg := iHTTPConfig(ts.URL)
 
 	iCreateTarget(
 		t, s.MainDB, targetID, s.WebhookID, name,
-		database.TargetTypeHTTP, cfg, 5,
+		database.TargetTypeHTTP, ts.URL, 5,
 	)
 
 	event := iSeedEvent(
@@ -366,7 +365,7 @@ func tRetryChainSetup(
 	body := event.Body
 
 	return iTask(
-		d, event, s.WebhookID, targetID, name, cfg, 5, 2, &body,
+		d, event, s.WebhookID, targetID, name, ts.URL, 5, 2, &body,
 	), targetID
 }
 
@@ -527,7 +526,7 @@ func TestRecoverSingleRetry_TargetUnreadable_LeavesDeliveryAlone(
 	iCreateTarget(
 		t, s.MainDB, targetID, s.WebhookID, "healthy",
 		database.TargetTypeHTTP,
-		iHTTPConfig("http://example.com/hook"), 5,
+		"http://example.com/hook", 5,
 	)
 
 	event := iSeedEvent(