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.

templates/source_detail.html

@@ -110,6 +110,14 @@
                     <div x-show="targetType === 'http'">
                         <input type="url" name="url" placeholder="https://example.com/webhook" :disabled="targetType !== 'http'" class="input text-sm">
                     </div>
+                    <div x-show="targetType === 'http'">
+                        <textarea name="headers" rows="3" placeholder="Authorization: Bearer ..." :disabled="targetType !== 'http'" class="input text-sm"></textarea>
+                        <p class="text-xs text-gray-500 mt-1">Optional request headers, one <code>Name: value</code> per line, sent with every delivery.</p>
+                    </div>
+                    <div x-show="targetType === 'http'" class="flex gap-2 items-center">
+                        <label class="text-sm text-gray-700">Timeout (seconds, blank = default):</label>
+                        <input type="number" name="timeout" min="0" max="300" :disabled="targetType !== 'http'" class="input text-sm w-24">
+                    </div>
                     <div x-show="targetType === 'http'" class="flex gap-2 items-center">
                         <label class="text-sm text-gray-700">Max retries (0 = fire-and-forget):</label>
                         <input type="number" name="max_retries" value="0" min="0" max="20" class="input text-sm w-24">
@@ -138,6 +146,7 @@
                             {{else}}
                             <span class="badge-error">Inactive</span>
                             {{end}}
+                            <a href="/source/{{$.Webhook.ID}}/targets/{{.ID}}/edit" class="text-xs text-gray-500 hover:text-primary-600" title="Edit">Edit</a>
                             <form method="POST" action="/source/{{$.Webhook.ID}}/targets/{{.ID}}/toggle" class="inline">
                                 <input type="hidden" name="csrf_token" value="{{$.CSRFToken}}">
                                 <button type="submit" class="text-xs text-gray-500 hover:text-primary-600" title="{{if .Active}}Deactivate{{else}}Activate{{end}}">