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/handlers/event_log.go

@@ -13,8 +13,8 @@ import (
 
 // DeliveryView is the display-safe projection of a delivery
 // for the event log page. Its target is a TargetView, so the
-// stored configuration blob — which holds the target's
-// credential — has no path to the template.
+// target's URL and headers — which hold the target's
+// credential — have no path to the template.
 type DeliveryView struct {
 	ID     string
 	Status database.DeliveryStatus
@@ -126,7 +126,7 @@ func (h *Handlers) HandleSourceLogs() http.HandlerFunc {
 // loadTargetMap loads targets into a map of display-safe
 // views keyed by target ID, each paired with its redactor.
 // The projection happens here so that no caller can hand a
-// raw target, configuration blob and all, to a template: the
+// raw target, URL and headers and all, to a template: the
 // raw rows do not leave this function.
 //
 // The load is Unscoped because deleting a target only soft