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/lib/actionErrors.test.ts

@@ -87,27 +87,6 @@ describe("notifyActionError", () => {
     });
   });
 
-  it("tells a demo user that the demo account is the reason", async () => {
-    // `require_human`'s refusal was a plain English string, so round 5's fix — discard a
-    // plain-string detail, show the caller's translated fallback — discarded the only explanation
-    // the demo user got: a generic "couldn't pin it" with no hint why (round 6). The gate answers
-    // a structured code now, and `localizeDetail` has a branch for it in both locales.
-    vi.spyOn(globalThis, "fetch").mockResolvedValue(
-      new Response(JSON.stringify({ detail: { code: "demo" } }), {
-        status: 403,
-        headers: { "content-type": "application/json" },
-      }),
-    );
-    const err = await api.addComfortPin("video", "v1").catch((e: unknown) => e);
-    notifyMock.mockClear();
-    notifyActionError(err, "GENERIC FALLBACK");
-    expect(notifyMock).toHaveBeenCalledWith({
-      level: "error",
-      title: "GENERIC FALLBACK",
-      message: i18n.t("errors.demo"),
-    });
-  });
-
   it("still prefers a STRUCTURED reason, which the request layer has already localized", () => {
     // `req` runs `{code, reason}` through localizeDetail, so `detail` is in the reader's language
     // AND more specific than anything a status can say — quota numbers, which row is gone.