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/archive/rotation.go

@@ -1,7 +1,6 @@
 package archive
 
 import (
-	"encoding/json"
 	"errors"
 	"fmt"
 	"os"
@@ -54,35 +53,10 @@ func ValidateRotation(rotation string) error {
 	}
 }
 
-// ParseRotation reads the rotation from a database target's
-// config JSON. An empty config or an empty rotation is none.
-func ParseRotation(configJSON string) (string, error) {
-	if configJSON == "" {
-		return archiveRotationNone, nil
-	}
-
-	var cfg DatabaseTargetConfig
-
-	err := json.Unmarshal([]byte(configJSON), &cfg)
-	if err != nil {
-		return "", fmt.Errorf("parsing database target config: %w", err)
-	}
-
-	err = ValidateRotation(cfg.Rotation)
-	if err != nil {
-		return "", err
-	}
-
-	if cfg.Rotation == "" {
-		return archiveRotationNone, nil
-	}
-
-	return cfg.Rotation, nil
-}
-
 // archivePeriod returns the period, in UTC, that a rotation puts an
 // event received at receivedAt in: "2026-10" for monthly,
-// "2026-10-01" for daily, "2026-10-01-19" for hourly, and "" for none.
+// "2026-10-01" for daily, "2026-10-01-19" for hourly, and "" for none
+// or an empty rotation.
 func archivePeriod(rotation string, receivedAt time.Time) string {
 	switch rotation {
 	case archiveRotationMonthly:
@@ -115,16 +89,11 @@ func PathAt(
 	webhook *database.Webhook,
 	target *database.Target,
 	receivedAt time.Time,
-) (string, error) {
-	rotation, err := ParseRotation(target.Config)
-	if err != nil {
-		return "", err
-	}
-
+) string {
 	return archivePeriodPath(
 		Path(dbMgr, webhook, target),
-		archivePeriod(rotation, receivedAt),
-	), nil
+		archivePeriod(target.ArchiveRotation, receivedAt),
+	)
 }
 
 // archiveFiles lists the database target's archive files that exist,