Encapsulate job pool behind a single handle (job_pool_*)

Replace the four-parallel-nameref job-pool API (pids/statuses/labels/failed,
each needing its own `shellcheck disable=SC2034` at every call site) with a
single pool handle: a name prefix whose four backing variables
(${pool}_pids/_statuses/_labels/_failed) are derived on demand by the helpers.
Bash can't nest indexed arrays in an associative array, so a prefixed-handle
with declare -g backing vars is the simplest pure-nameref encoding (no eval).

New public API:
  job_pool_init <pool>
  job_pool_submit <pool> <label> <cmd...>
  job_pool_wait <pool>            # returns 1 if any job failed

Migrate all callers (scalephotos, create_all_photo_derivatives,
render_album_pages, render_view_redirects, render_filter_pages) to the handle.
queue_preview_page_render_job / queue_album_view_render_job /
_album_record_view_photo / _stats_enqueue_filter_album now take one pool arg
instead of four names.

Drop the dead wrappers and unused parameterization: wait_for_image_job_slot,
wait_for_template_render_job_slot and their _jobs variants only ever passed
IMAGE_JOBS, so the max_jobs parameter is gone and throttling is fixed at
IMAGE_JOBS inside the pool. Also removes wait_for_album_view_render_jobs (now
just job_pool_wait).

Throttling (max IMAGE_JOBS concurrent), failure detection and failed-job
propagation are unchanged; the parallel-throttling and failure-logging tests
pass unmodified. SC2034 disable-comments across the touched files drop 29->7.

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

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