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/delivery/recovery_durability_test.go

@@ -357,7 +357,7 @@ func TestFailedResultWriteLeavesDeliveryRecoverable(
 		Target: database.Target{
 			Name:        "unwritable",
 			Type:        database.TargetTypeHTTP,
-			Config:      iHTTPConfig(ts.URL),
+			URL:         ts.URL,
 			MaxAttempts: 1,
 		},
 	}
@@ -448,7 +448,7 @@ func TestFailedResultWriteWithRetriesLeavesDeliveryRecoverable(
 				Target: database.Target{
 					Name:        "unwritable",
 					Type:        database.TargetTypeHTTP,
-					Config:      iHTTPConfig(ts.URL),
+					URL:         ts.URL,
 					MaxAttempts: 3,
 				},
 			}