253 more dead alphatheta manual links: the whole downloads/manuals tree is gone, not just dj-effectors

[jeffreylouden]

Surfaced while fixing #654. That issue was scoped to the 7 Pioneer DJ effector
entries the broken-urls label on #653 happened to touch. The rot is not
scoped to dj-effectors.

Evidence

downloads.support.alphatheta.com/manuals/ itself 404s, as does every
subdirectory. data/ holds 260 links under that tree:

path segment	links
dj-controllers	63
dj-mixers	46
headphones	45
dj-players	37
monitor-speakers	25
pa-speakers	13
all-in-one-dj-systems	12
accessories	8
turntables	5
music-production	3
software-interfaces	2
dj-effectors	1 (7 before #654)

A 12-URL sample spread across those families, fetched with a browser
User-Agent and with the ?page= fragment removed, returned 404 on every one.
https://downloads.support.alphatheta.com/ itself returns 200, so the host is
healthy and the tree is what was removed.

#654 fixed the 7 effector entries, leaving 253.

Where the manuals went

AlphaTheta moved manual content into help-center articles. The resolution path
is mechanical:

1. The entry's existing support.alphatheta.com/products/<product_id> link is
   the product's support page. 210 of the 260 affected files already carry
   one; the remaining 50 need another route (5 carry a
   support.pioneerdj.com/hc/... link that redirects to the same place).
2. Fetch https://support.alphatheta.com/en-US/products/<product_id> and read
   its embedded RSC payload. Manual articles live under ?section=23; the
   record's article_id (not its id, which is the translation id) is the
   one that resolves.
3. The article URL is https://support.alphatheta.com/articles/<article_id>,
   which 200s and redirects to the locale-prefixed form.

Some products also carry a dedicated Specifications article (EFX-1000's is
24199943617177, holding exactly what the dead ?page=28 anchor pointed at),
which is a like-for-like replacement preserving the link title. Where none
exists, the instruction-manual article is the replacement and the link should
be retitled, since no per-page anchor exists to link any more. That is the
convention #654 established.

Scope note

This is 253 files. It is worth doing as a scripted pass with the resolution
above rather than by hand, and worth deciding deliberately rather than letting
it ride along with an unrelated PR, which is why it is filed instead of folded
into #672.

The false-positive noise in validate-urls on support.alphatheta.com/products/…
and tascam.com (200 to a browser, 403/404 to the checker) noted in #654 is
still a separate concern about the checker.