Add a target edit form and reachable header/timeout fields (closes #127)

A target's configuration was write-once: `internal/server/routes.go`
registered create, toggle and delete for targets but no edit route, so
correcting a typo in a destination meant deleting the target and
recreating it. Masking the stored value made that unrecoverable from
the UI.

Headers and timeout were worse than write-once. `HTTPTargetConfig` has
carried `Headers` and `Timeout` and the delivery path has honoured both,
but `buildURLTargetConfig` only ever wrote `{"url":...}` and no form
offered either field, so a destination needing an `Authorization` header
could not be configured through the UI at all.

Both paths now build their configuration through `buildTargetConfig`, so
an edited destination is SSRF-validated exactly as a new one is. The
destination check lives in one helper that both reach, leaving the guard's
entry point untouched.

The edit form pre-fills the stored destination and header values in full.
That is the one intentional exception to the masking rule, narrowed by
the route it lives on: `RequireAuth`, `NoCache`, and the webhook's
ownership check. `delivery.TargetView` is unchanged, so every other page
still masks.

A target's type stays fixed at creation: each type stores a different
configuration shape and its delivery history is recorded against the row,
so changing it is really a different target.

Header and timeout input that could not be delivered as written is
rejected rather than stored: a malformed line, an invalid name, a control
character in a value, a repeated name, a header the delivery engine
overwrites regardless, or a timeout that is not a whole number of seconds
within the ceiling. Storing input that provably never reaches the wire
would report a configuration that did not take effect.

internal/server/routes.go

@@ -214,6 +214,20 @@ func (s *Server) setupSourceRoutes() {
 			s.h.HandleEntrypointToggle(),
 		)
 		r.Post("/targets", s.h.HandleTargetCreate())
+		// The edit form is the one page that renders a target's
+		// destination URL and header values in full; see
+		// delivery.TargetConfigForm. It belongs to this group for
+		// its RequireAuth and NoCache, which are what keep that
+		// exception from reaching an unauthenticated request or a
+		// shared cache.
+		r.Get(
+			"/targets/{targetID}/edit",
+			s.h.HandleTargetEdit(),
+		)
+		r.Post(
+			"/targets/{targetID}/edit",
+			s.h.HandleTargetEditSubmit(),
+		)
 		r.Post(
 			"/targets/{targetID}/delete",
 			s.h.HandleTargetDelete(),