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/database/schema_test.go

@@ -87,13 +87,18 @@ func TestMainSchemaHoldsEveryModel(t *testing.T) {
 		Active:      true,
 	})
 	assertRoundTrip(t, db, &database.Target{
-		BaseModel:   newBaseModel(),
-		WebhookID:   uuid.New().String(),
-		Name:        "target",
-		Type:        database.TargetTypeHTTP,
-		Active:      true,
-		Config:      `{"url":"https://example.com/"}`,
-		MaxAttempts: 3,
+		BaseModel:       newBaseModel(),
+		WebhookID:       uuid.New().String(),
+		Name:            "target",
+		Type:            database.TargetTypeHTTP,
+		Active:          true,
+		URL:             "https://example.com/",
+		Headers:         map[string]string{"X-Api-Key": "key"},
+		TimeoutSeconds:  30,
+		ForwardQuery:    true,
+		ArchiveExpiry:   720 * time.Hour,
+		ArchiveRotation: "daily",
+		MaxAttempts:     3,
 	})
 }