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/server/routes_test.go

@@ -414,7 +414,7 @@ func (e *testEnv) seedTarget(
 		Name:        "routed-target",
 		Type:        database.TargetTypeHTTP,
 		Active:      true,
-		Config:      `{"url":"http://93.184.216.34/hook"}`,
+		URL:         "http://93.184.216.34/hook",
 		MaxAttempts: 1,
 	}