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/webhook_stats_test.go

@@ -153,7 +153,7 @@ func seedStatsHistory(
 
 	first := seedConfiguredTarget(
 		t, db, wh.ID, database.TargetTypeHTTP,
-		`{"url":"`+replayTargetURL+`"}`,
+		database.Target{URL: replayTargetURL},
 	)
 	second := seedTarget(t, db, wh.ID, database.TargetTypeLog)