Use one name for each thing the UI shows (closes #399)

The UI gave one thing several names. The `database` type is now Archive in the type list, on its badge and on the edit page, and its settings read "Archive expiry" and "Archive rotation" everywhere. The retry field is "Delivery attempts", with help text, errors and the target list saying it counts every attempt; a stored 0 shows as one attempt. The target list uses one capitalisation. The navbar says "Sign out", the sign-in page "Sign in", and the resubmit notice "webhook" instead of "source". The README follows. Stored values, their meaning, routes and form field names are unchanged.

Model: opus-5-5

templates/login.html

@@ -1,6 +1,6 @@
 {{template "base" .}}
 
-{{define "title"}}Login - Webhooker{{end}}
+{{define "title"}}Sign in - Webhooker{{end}}
 
 {{define "content"}}
 <div class="min-h-screen flex items-center justify-center py-12 px-4">
@@ -52,7 +52,7 @@
                     >
                 </div>
 
-                <button type="submit" class="btn-primary w-full py-3">Sign In</button>
+                <button type="submit" class="btn-primary w-full py-3">Sign in</button>
             </form>
         </div>
     </div>