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/components/Sidebar.tsx

@@ -21,7 +21,6 @@ import Combobox from "./ui/Combobox";
 import MatchToggle from "./ui/MatchToggle";
 import { useFeedFilters, useFeedFiltersActions } from "./feedFiltersContext";
 import { useChannelsActions } from "./channelsContext";
-import { useMe } from "../lib/useMe";
 import { useTags } from "../lib/useTags";
 import { compareText } from "../lib/collate";
 
@@ -51,7 +50,6 @@ export default function Sidebar({
 }) {
   const { t, i18n } = useTranslation();
   const lang = i18n.language;
-  const isDemo = useMe().is_demo;
   const { openModal } = useSavedViews();
   const filters = useFeedFilters();
   const { setFilters } = useFeedFiltersActions();
@@ -245,14 +243,14 @@ export default function Sidebar({
       // The feed widgets are simple label + control rows, not collapsible islands.
       collapsible={false}
       // Saved ("personal") views: the header icon opens a quick-pick popover, the title opens the
-      // full manager modal. Hidden for the demo account (no per-user views). Feed rail only.
+      // full manager modal. Feed rail only.
       iconExpanded={
-        !isDemo && source === "feed" ? (
+        source === "feed" ? (
           <SavedViewsPopover trigger={<SlidersHorizontal className="w-4 h-4" />} />
         ) : undefined
       }
-      onTitleClick={!isDemo && source === "feed" ? openModal : undefined}
-      titleActionLabel={!isDemo && source === "feed" ? t("views.manageTitle") : undefined}
+      onTitleClick={source === "feed" ? openModal : undefined}
+      titleActionLabel={source === "feed" ? t("views.manageTitle") : undefined}
     >
       {tagManagerOpen && (
         <TagManager onClose={() => setTagManagerOpen(false)} onFocusChannel={onFocusChannel} />