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

Adding or editing a database target now offers the archive expiry
choices of the new webhook page (never, 1h, 12h, 24h, 30d, 90d, 365d)
in place of a text field. The list is defined once, in
internal/handlers/archive_expiry.go, and all three forms render it. The
edit form starts on the expiry it shows: the stored one, or the
submitted one after a refused save. One that is not among the choices
is listed first as its own entry, so saving unchanged keeps it. The
target list shows the expiry in plain units, such as "30 days" or
"12 hours", or "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.