Fix translate-and-adapt for in-app articles (silent failure)

[mircealungu]

Summary

A user opened a foreign-language article in the reader, chose Translate to my level, saw "Translating…", and then nothing happened. Reproduced from logs:

Failed to detect source language: Readability failed to parse article
Translation failed for https://zeeguu.org/read/article?id=4604129:
  cannot access local variable 'article_obj' where it is not associated with a value
POST //translate_and_adapt_article ... 500

Two compounding bugs (this PR is the backend half):

1. /translate_and_adapt_article re-downloaded the article's stored URL through readability. For articles whose stored URL is a Zeeguu reader link (zeeguu.org/read/article?id=…, a JS SPA), the download failed — and the code then fell through and used the never-assigned article_obj, raising UnboundLocalError → HTTP 500.
2. (Web PR) the reader swallowed the error and just closed the modal.

Changes

• Endpoint now accepts article_id and translates the already-stored article content directly — no re-download, no readability dependency — and reuses the stored image.
• The URL path (external shares) is kept, but now abort(502) cleanly instead of crashing when a fetch fails.

Companion PR

• web: claude/translate-adapt-stored-article (passes article_id, graceful fallback)

Test plan

• Open a foreign-language article in the reader (deeplink) and tap Translate to my level → navigates to the translated/adapted version
• Article whose stored URL is a Zeeguu reader link (e.g. id 4604129) translates successfully
• External share of a normal URL still translates (URL path)
• Unfetchable URL returns 502, not 500

🤖 Generated with Claude Code

[github-actions]

ArchLens - No architecturally relevant changes to the existing views