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/db/migrations/main/001_schema.sql

@@ -79,7 +79,12 @@ CREATE TABLE targets (
     name text NOT NULL,
     type text NOT NULL,
     active numeric DEFAULT true,
-    config text,
+    url text,
+    headers text,
+    timeout_seconds integer,
+    forward_query numeric,
+    archive_expiry integer,
+    archive_rotation text,
     max_attempts integer NOT NULL,
     PRIMARY KEY (id),
     CONSTRAINT fk_webhooks_targets