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/app/routes/channels.py

@@ -12,7 +12,7 @@ from sqlalchemy.orm import Session
 from app.collation import collated, sorted_words
 from app.mailtext import lang_for_user
 from app import logs, quota
-from app.auth import admin_user, current_user, require_human, require_write_scope
+from app.auth import admin_user, current_user, require_write_scope
 from app.db import get_db
 from app.models import (
     LIVE_OR_UPCOMING,
@@ -163,8 +163,8 @@ def discover_channels(
     can judge a channel BEFORE subscribing — but we do NOT pull its videos. Most-present
     channels come first; subscribing is a separate action."""
     # Resolve & cache the user's own channel id so it can be excluded — you don't subscribe
-    # to yourself. Lazy + cached; needs OAuth, so demo / token-less users just skip it.
-    if user.yt_channel_id is None and user.token is not None and not user.is_demo:
+    # to yourself. Lazy + cached; needs OAuth, so token-less users just skip it.
+    if user.yt_channel_id is None and user.token is not None:
         try:
             with (
                 quota.attribute(user.id, quota.QuotaAction.CHANNELS_DISCOVER),
@@ -192,8 +192,7 @@ def discover_channels(
     # (fresh subscriber counts/thumbnails carry through without a re-query). Only the title can
     # change, which affects the tie-break order → re-sort in Python instead of re-hitting the DB.
     need = [ch.id for ch, _, _ in rows if ch.details_synced_at is None][:DISCOVERY_ENRICH_CAP]
-    # Skip the quota-spending enrich for the demo account (it just sees the stub metadata).
-    if need and not user.is_demo:
+    if need:
         try:
             with (
                 quota.attribute(user.id, quota.QuotaAction.CHANNELS_DISCOVER),
@@ -311,12 +310,12 @@ def channel_detail(
     Lazily fills the About metadata (view count, join date, banner, links) the first time it's
     needed (a stub) and refreshes it once it is stale (`DETAILS_MAX_AGE`), which is how a channel nobody subscribes to (an explored one, outside the
     subscription import) gets a banner that still resolves. The enrich uses the API key (1 unit,
-    cached after), so it's skipped for the quota-less demo account."""
+    cached after)."""
     channel = get_or_404(db, Channel, channel_id, what="channel")
     # No `published_at is None` sentinel: a terminated channel never gets one, so it re-fetched on
     # every view; rows enriched before that field existed are long past DETAILS_MAX_AGE anyway.
     stale = details_stale(channel, datetime.now(timezone.utc))
-    if stale and not user.is_demo:
+    if stale:
         try:
             with (
                 quota.attribute(user.id, quota.QuotaAction.CHANNELS_DISCOVER),
@@ -389,7 +388,7 @@ def channel_detail(
 def explore_channel(
     channel_id: str,
     page_token: str | None = None,
-    user: User = Depends(require_human),
+    user: User = Depends(current_user),
     db: Session = Depends(get_db),
 ) -> dict:
     """Open an un-subscribed channel for browsing. Records the exploration (a per-user,
@@ -397,7 +396,7 @@ def explore_channel(
     drives the later cleanup), flags the channel ephemeral unless someone organically follows
     it, then ingests one page of its uploads (newest-first) marked via_explore + enriched.
     Returns the next page token so the page can "load more". Guarded against the shared
-    backfill quota reserve; blocked for the demo account (it spends shared quota)."""
+    backfill quota reserve."""
     channel = get_or_404(db, Channel, channel_id, what="channel")
     if _is_blocked(db, user, channel_id):
         raise refused(403, "channel_blocked")
@@ -493,10 +492,7 @@ def update_channel(
         sub.priority = body.priority
     if body.hidden is not None:
         sub.hidden = body.hidden
-    # The demo account is SHARED, so it must not persist deep_requested (a quota-relevant flag) onto
-    # the shared subscription — skip the write entirely for demo, which also keeps deep_turned_on
-    # False so the immediate backfill below can never fire for it.
-    if body.deep_requested is not None and not user.is_demo:
+    if body.deep_requested is not None:
         # Opt this channel into (or out of) full-history backfill. The deep scheduler
         # picks the flag up on its next run; turning it off won't undo already-fetched
         # videos, it just stops further deep paging if nobody else still wants it.
@@ -507,7 +503,7 @@ def update_channel(
 
     # Opting a channel into full history should give an immediate feed: if we haven't even
     # fetched its recent uploads yet, do that one channel now (cheap) instead of waiting for
-    # the scheduler. (deep_turned_on is only ever True for a non-demo user — see the guard above.)
+    # the scheduler.
     if deep_turned_on and channel is not None and channel.recent_synced_at is None:
         with quota.attribute(user.id, quota.QuotaAction.VIDEOS_BACKFILL_RECENT):
             run_recent_backfill(db, [channel], max_channels=1)