feat!: remove the shared demo account

The demo account — one shared user that whitelisted emails entered from
the login page, reset on a schedule — goes end to end (owner decision,
E34): demo.py and the reset job, users.is_demo and demo_whitelist
(migration 0098 deletes the demo user, its audit rows, the reset job's
override and notices, then drops the column and table; the downgrade
restores the empty schema), the /auth/demo entry, the admin whitelist and
reset, the Demo tab, the banner, the login page's "Try the demo", and
every is_demo branch in routes, userscope, scheduler and the UI.

require_human only ever refused the demo account, so its 87 uses become
current_user (owner-approved); require_write_scope keeps its own check.
The login page's "or" divider now shows only beside the Google button:
without the demo entry under it, an instance without Google would have
said "or" to nothing.

A contract test (removedFeatures) fails on the word anywhere in the
backend, frontend, e2e or scripts — seen red on a planted hit. Size
ceilings tightened for the modules that shrank; admin.py is back under
the limit.

frontend/src/i18n/locales/en/errors.json

@@ -1,7 +1,6 @@
 {
   "title": "Couldn't complete that",
   "generic": "The server couldn't carry out that action. Please try again.",
-  "demo": "This isn't available in the demo account",
   "validation": "Some of the values you entered weren't valid. Please check them and try again.",
   "tooLarge": "That was too large for the server to accept",
   "server": "Server error",
@@ -45,8 +44,7 @@
     "not_applicable": "This account has nothing left to confirm",
     "not_approved": "Only an approved address can be invited",
     "email_off": "No mail was sent: this instance has no working SMTP setup. Check Configuration → Email / SMTP.",
-    "send_failed": "The mail couldn't be sent: the mail server was unreachable, or it refused the login or the message. Check Configuration → Email / SMTP, then try again.",
-    "demo": "The demo account is shared, so it can't get a set-password link"
+    "send_failed": "The mail couldn't be sent: the mail server was unreachable, or it refused the login or the message. Check Configuration → Email / SMTP, then try again."
   },
   "reason": {
     "unreachable": "The server couldn't be reached. Try again.",
@@ -132,10 +130,6 @@
     "collection_pending_sync": "The collection changed in Plex, but refreshing the local copy failed — it catches up on the next sync",
     "collection_read_only": "This collection is read-only",
     "collection_smart": "Smart and auto-list collections can't be made editable",
-    "demo_activate": "The demo account can't be activated",
-    "demo_delete": "The demo account can't be deleted",
-    "demo_role": "The demo account's role can't be changed",
-    "demo_suspend": "The demo account can't be suspended",
     "download_not_ready": "This download isn't ready",
     "edit_unfinished": "Only a finished download can be edited",
     "filters_not_object": "The view's filters are malformed",