Offer archive expiry choices on the target forms, show plain units (closes #396)

A database target's archive expiry was typed by hand as never or a raw duration such as 720h, and the target list showed it back raw. Adding or editing a database target now offers the new-webhook page's list of choices (never, 1h, 12h, 24h, 30d, 90d, 365d), defined once and shared by all three forms. The edit form starts on the stored expiry, or on the submitted one after a refused save; a stored value outside the choices is listed under its own value, so saving unchanged keeps it. The target list shows the expiry in plain units: "30 days", "12 hours", "never".

Model: opus-5-5

README.md

@@ -1721,8 +1721,11 @@ events should be forwarded.
   own archive database
   (`archive-{webhook_name}-{target_name}-{target_uuid}.db`) for long-term
   retention, with an optional creation-validated expiry (default: keep
-  forever). No external delivery and no retries; an archive write
-  failure fails the delivery. See the database target section under
+  forever). The new webhook form, the add target form and the target edit
+  form all offer the same expiries: never, 1h, 12h, 24h, 30d, 90d or 365d.
+  The target list shows the expiry in plain units, such as "30 days". No
+  external delivery and no retries; an archive write failure fails the
+  delivery. See the database target section under
   "Per-Webhook Event Databases" for the full semantics.
 - **`log`** — Write the event to the application log (stdout). Useful
   for debugging.