vn0 parallelize preview pages and navigation redirects

Only per-photo view/detail pages were backgrounded; preview pages
(page-*.html) and navigation redirects rendered serially in the main
loop, serializing album generation.

- render_full_preview_page assembles a complete preview page (header,
  optional prev link, ordered thumbnails, next/footer) in one call so
  the per-page appends stay sequential; queue_preview_page_render_job
  backgrounds each whole page via the shared template render pool,
  throttled to IMAGE_JOBS.
- render_album_pages now materializes the full deterministic page
  layout up front (album_page_records) and enqueues both view jobs and
  one preview-page job per page into the same pool; per-photo
  bookkeeping moved to _album_record_view_photo.
- render_view_redirects enqueues one job per view page
  (render_page_view_redirects) and blocks until all redirects exist,
  preserving its synchronous contract while parallelizing the work.
- Removed obsolete advance_album_preview_page and
  render_album_page_thumbnail.

Parallelism only affects timing: seeded random choices are
context-keyed, so output is byte-identical across runs. A failed
preview or redirect job flips render_failed and makes generation exit
non-zero, matching the view-job failure contract. The IMAGE_JOBS cap
still bounds concurrent renders.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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