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.

backend/tests/test_channel_backfill.py

@@ -1,5 +1,5 @@
 """R14 backend coverage: the channel-detail Normal/Shorts/Live counts (moved off the manager table),
-the update_channel -> immediate-backfill wiring (incl. the demo guard), and the interactive
+the update_channel -> immediate-backfill wiring, and the interactive
 "get full history" guard that keeps it from racing the scheduler's cursor or draining the budget."""
 
 from datetime import datetime, timezone
@@ -89,7 +89,7 @@ def test_channel_detail_hidden_defaults_false_when_not_subscribed(db, monkeypatc
     assert resp.json()["hidden"] is False
 
 
-def _setup_channel_and_sub(db, *, email, is_demo=False):
+def _setup_channel_and_sub(db, *, email):
     ch = Channel(
         id="UC_wire",
         title="Wire",
@@ -98,7 +98,7 @@ def _setup_channel_and_sub(db, *, email, is_demo=False):
         uploads_playlist_id="UU_wire",
     )
     db.add(ch)
-    user = User(email=email, is_demo=is_demo)
+    user = User(email=email)
     db.add(user)
     db.flush()
     db.add(Subscription(user_id=user.id, channel_id=ch.id, deep_requested=False))
@@ -117,7 +117,7 @@ def _patch_deep_requested(db, monkeypatch, override_dep, user):
     return resp, calls
 
 
-def test_deep_request_schedules_immediate_backfill_for_non_demo(db, monkeypatch, override_dep):
+def test_deep_request_schedules_immediate_backfill(db, monkeypatch, override_dep):
     ch, me = _setup_channel_and_sub(db, email="wire@example.com")
     resp, calls = _patch_deep_requested(db, monkeypatch, override_dep, me)
     assert resp.status_code == 200, resp.text
@@ -126,16 +126,6 @@ def test_deep_request_schedules_immediate_backfill_for_non_demo(db, monkeypatch,
     assert calls == [(ch.id, me.id)]
 
 
-def test_deep_request_does_not_schedule_immediate_backfill_for_demo(db, monkeypatch, override_dep):
-    # The SHARED demo account must not be able to kick immediate full backfills (budget drain).
-    _ch, demo = _setup_channel_and_sub(db, email="demo@example.com", is_demo=True)
-    resp, calls = _patch_deep_requested(db, monkeypatch, override_dep, demo)
-    assert resp.status_code == 200, resp.text
-    assert calls == [], "demo must not schedule an immediate backfill"
-    # …and deep_requested is not persisted on the shared demo subscription either.
-    assert resp.json()["deep_requested"] is False
-
-
 class TestImmediateBackfillGuard:
     """backfill_channel_now coordinates with the paced scheduler through _backfill_in_progress (per-
     channel mutual exclusion, both directions) + _immediate_running (a cap on interactive runs so a