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/database/event_db_isolation_test.go

@@ -28,7 +28,7 @@ func TestEventDBCreateOmitsAssociations(t *testing.T) {
 		WebhookID:   webhookID,
 		Name:        "leaky-target",
 		Type:        database.TargetTypeSlack,
-		Config:      `{"webhookUrl":"https://hooks.example/secret"}`,
+		URL:         "https://hooks.example/secret",
 		MaxAttempts: 1,
 	}
 	target.ID = uuid.New().String()