Fix five findings from a security assessment of the plugin

The assessment looked for three things: whether students can reach each
other's bookings, whether payment settings can be dodged, and whether the
plugin opens a way into the rest of the install. The student-isolation and
payment paths held up. These are what did not.

- The front-end login form told WordPress not to work out whether the site
  was secure, so on HTTPS every student's session cookie was issued without
  the Secure flag. wp_signon() only derives it from is_ssl() when the second
  argument is left at its default; an explicit false reads like "no
  preference" and is not.

- The update check took whatever download URL the release API returned and
  handed it to core, which unpacks it over the installed plugin. The package
  must now be https on git.unsupervised.ca exactly, compared on the parsed
  host so a lookalike name cannot pass.

- Uninstalling dropped 2 of 14 tables and left the Stripe secret and webhook
  signing key in wp_options. Removal is now a choice made in advance on
  Access -> Plugin removal: records are kept unless the owner opts in (with a
  typed confirmation), while credentials and the borrowed core registration
  settings go every time.

- Open registration switches on the site-wide users_can_register and makes
  Student the default role, arming any other signup form on the site to mint
  students who could book and be billed immediately. The pending state is now
  decided once, on user_register, rather than by whichever form created the
  account.

- Cancel and withdraw answered "not yours" differently from "does not exist",
  which let a signed-in student enumerate the studio's bookings. Both now
  give the same 404.

Co-Authored-By: Claude Opus 5 <[email protected]>

docs/features/group-classes.md

@@ -136,10 +136,12 @@ Self-withdrawal is bounded by the class's **withdrawal deadline** (the instructo
 implicit default — a class with no deadline set stays open to withdrawal for its
 whole life. Past the deadline `POST /enrollments/{id}/withdraw` rejects the request
 with `403 withdrawal_closed`, and the class card shows "Withdrawal has closed —
-contact the studio to withdraw." in place of the Withdraw button. The endpoint also
-returns `404 not_found` for an unknown enrolment and `403 forbidden` when the
-enrolment is not the caller's own; a withdrawal of an already-cancelled enrolment is
-idempotent.
+contact the studio to withdraw." in place of the Withdraw button. An enrolment that
+does not exist and one that is not the caller's own both return the same
+`404 not_found`, deliberately: two different answers would let any signed-in student
+walk the id space and count the studio's enrolments, and there is nothing they could
+do with either answer. A withdrawal of an already-cancelled enrolment is idempotent.
+`POST /bookings/{id}/cancel` makes the same trade for the same reason.
 
 The deadline only bounds student **self**-withdrawal. A studio admin can withdraw a
 student at any time from the **student detail page** (`Auth\StudentActions::withdrawEnrollment`),