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/backendIds.contract.test.ts

@@ -228,7 +228,7 @@ describe("backend ids have their names in both languages", () => {
     expect(jobs).toEqual(expect.arrayContaining(["rss_poll", "audit_gc"]));
     expect(auditActions).toEqual(expect.arrayContaining(["config_set", "audit_clear"]));
     expect(quotaActions.length).toBeGreaterThan(0);
-    expect(adminCodes).toEqual(expect.arrayContaining(["activate", "setpw"]));
+    expect(adminCodes).toEqual(expect.arrayContaining(["activate", "resend"]));
     expect(failDetails).toEqual(expect.arrayContaining(["facebook.login_wall"]));
     expect(goneWhats).toEqual(expect.arrayContaining(["playlist", "download", "job"]));
     expect(opaqueWhats).toEqual([]);