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/plugin-self-update.md

@@ -47,9 +47,21 @@ update for a same-slug plugin and makes core fire the
    `us_schedular_latest_release` transient for 6 hours.
 3. Strips the leading `v` from the tag and compares against `USC_VERSION`
    with `version_compare`; PHP orders `1.0.0-rc.2 < 1.0.0` correctly.
-4. When newer, returns the release's first `.zip` asset as the update
-   package. Core takes over from there: Plugins-screen notice, one-click
-   update, and WP-Cron auto-updates if enabled.
+4. When newer, returns the release's first `.zip` asset **whose download URL
+   is `https` on `git.unsupervised.ca` itself** as the update package. Core
+   takes over from there: Plugins-screen notice, one-click update, and
+   WP-Cron auto-updates if enabled.
+
+   The host check is not ceremony. Whatever this returns is downloaded and
+   unpacked over the installed plugin, so the URL is executable code by
+   another name — and it arrives in a JSON body. An answer that is not really
+   the release server's (a hijacked hostname, a tampered response, a repo host
+   handing out a package hosted somewhere else) would otherwise install
+   arbitrary code on every site running the plugin, silently for anyone with
+   auto-updates on. The host must match exactly: `evil-git.unsupervised.ca`,
+   `git.unsupervised.ca.evil.test` and `cdn.git.unsupervised.ca` are all
+   refused, and so is plain `http`. An asset that fails the check is skipped
+   and the scan continues, so one bad asset does not hide a good one.
 5. When not newer — the site is current, or the lookup failed — returns a
    `no_update` payload (installed version, empty package). This keeps the
    plugin in core's `update_plugins` transient so core's `update-supported`