Fix top-genres banner overlap + polling backoff after 90%

[vanajmoorthy]

Two small dashboard follow-ups from PR #98's deferred list, plus refactor from review feedback.

1. Banner no longer collides with content; row tiles align

The "Still enriching" banner is absolute top-0 inset-x-0 z-10. Tiles WITH the banner had their content overlapped (page-count number partially covered, donut chart sat under the translucent yellow); sibling tiles in the same row had no offset and looked misaligned.

Fix: every tile inside an .enrichment-row grid reserves a banner-height zone via a single CSS rule:

.enrichment-row:has(.enrichment-banner) > * { padding-top: 1.75rem; }
@media (min-width: 768px) {
    .enrichment-row:has(.enrichment-banner) .enrichment-corner-chart {
        top: 1.75rem !important;
    }
}

The enrichment_banner.html partial now actually renders nothing when enrichment.pending is false (was relying on Alpine x-show), so :has() is a true signal. Four row containers gained the .enrichment-row marker class; the corner charts in top-genres/top-authors gained .enrichment-corner-chart. Replaces ten copies of {% if enrichment.pending %}pt-7{% endif %} from earlier iterations of this PR.

2. Polling backoff once enrichment crosses 90%

enrichmentPoller polled /api/enrichment-status/ on a fixed 5s interval. The last 10% of enrichment is mostly long-tail stragglers (books in retry/backoff), so the dashboard kept hammering the server every 5s for what could be several minutes.

Fix: when percent > 90, switch to a 12s interval. The threshold is monotonic — once we've slowed down, we never speed back up — so a non-monotonic percent (rounding, denominator shifts when a new book lands mid-pass) can't thrash the timer. poll() also guards on an inFlight flag so a hung fetch can't stack up behind a fresh interval.

Test plan

• docker-compose -f docker-compose.local.yml exec web poetry run python manage.py test → 343 passed
• pnpm run build → .enrichment-row:has(.enrichment-banner) present in built CSS
• Manual verification (4 scenarios):
  • Desktop, enrichment pending: tiles in banner rows align; chart sits below banner; banner doesn't cover the page-count number
  • Desktop, enrichment complete (after auto-reload): no banner, no extra padding, chart back at top-2
  • Mobile, enrichment pending: cards stack vertically; banner-bearing tile clears its banner; sibling tiles get the same pt-7 (extra empty space at top, harmless on stacked layout)
  • DevTools → Network: /api/enrichment-status/ fires every ~5s while percent ≤ 90, then every ~12s after