Clarify web UI terminology, copy, and the entrypoint URL (closes #57)

Unifies user-visible copy on "Webhook" (routes and URLs unchanged), drops
the placeholder Profile settings section, and adds a copy-to-clipboard
affordance for the entrypoint URL as progressive enhancement — the button
stays hidden unless both the target element and the Clipboard API resolve,
so no dead control appears without JavaScript and the URL stays selectable.

Retention copy now matches what the code does: deletion is permanent, 0
retains forever, and a blank field means the default on create or the
current value on edit. The permanent-deletion sentence is suppressed for a
retain-forever webhook, which the reaper exempts before computing a cutoff.

Template tests gained a render-completed assertion. Without it, a page that
aborted mid-render still satisfied assertions matching the already-flushed
prefix, because renderTemplate streams to the ResponseWriter (#123).

templates/sources_new.html

@@ -29,7 +29,7 @@
             <div class="form-group">
                 <label for="retention_days" class="label">Retention (days)</label>
                 <input type="number" id="retention_days" name="retention_days" value="{{.DefaultRetentionDays}}" min="0" class="input">
-                <p class="text-xs text-gray-500 mt-1">How long to keep event data. Enter 0 to retain events forever.</p>
+                <p class="text-xs text-gray-500 mt-1">A periodic cleanup permanently deletes events older than this, along with their delivery records. Enter 0 to retain events forever; leave blank to use the default of {{.DefaultRetentionDays}} days.</p>
             </div>
 
             <div class="flex gap-3">