Improve scraper's content extraction

[AJaccP]

🧠 Context

html_scraper.scrape() (src/ingestion/scrapers/html_scraper.py) does static-HTML extraction with httpx + trafilatura. It works for plain prose pages but misses a lot of what matters on Carleton CS pages. The known gaps (already noted in the file's comments) are:

• Content inside accordion / FAQ widgets is lost — often the most important info. Two distinct failure modes:
  1. Content hidden via aria-hidden="true" is dropped entirely — both the question and the answer disappear.
  2. On other FAQ layouts, the answer text is captured but the question isn't — so the answer has no context.
• Pages carry a lot of nav/header/footer/sidebar boilerplate that bloats chunks.
• PDF links and other links found on a page aren't surfaced usefully — they get dropped or mangled.

This ticket improves extraction quality across the page types we actually ingest, and captures PDF/links instead of losing them.

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🎯 Goals

1. Main-content extraction — scope extraction to the page's main content, dropping nav/header/footer/sidebars so chunks aren't full of boilerplate.
2. Recover accordion/FAQ content — after this ticket, for both failure modes above, the question and its answer both appear in the output and stay together as a coherent Q&A.
3. Capture links instead of losing them — surface PDF links as [PDF: <url>] markers so they ride into the content and can be cited. The domain schema already has a pointer source type (see the source_type argument on Repository.upsert_chunk / get_or_create_source) for representing PDFs/links if you choose to store them as their own entries. Other useful links should be preserved in the text.

🚧 Hard boundaries

• Do not crawl/follow links or download PDFs. Capture the PDF/link URL only — this keeps the knowledge base a deliberately curated set. No PDF text extraction.
• Chunking is unchanged. Leave the RecursiveCharacterTextSplitter config as-is — this ticket is about what text we extract, not how it's chunked.
• Don't populate section_heading. Leave it None; heading/structure extraction is a separate effort.

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🛠 Approach (investigation-first)

1. Spike first. Run the current scraper against a few representative pages (see test targets below) and look at exactly what's missing. Don't commit to an approach before you've seen the real HTML.
2. Try httpx → HTML preprocessing → trafilatura. The likely fix is preprocessing the HTML before extraction: select the main-content container, and un-hide / restructure the accordion markup so the hidden Q&A survives (the content is in the static HTML — trafilatura just discards hidden/widget nodes). Trafilatura already runs with include_links/include_tables; the gap is the hidden content, which is a preprocessing problem.
3. Fall back to BeautifulSoup if preprocessing + trafilatura can't get there. lxml is already installed (a good bs4 parser); add beautifulsoup4 via uv add beautifulsoup4 and commit the updated uv.lock (CI runs uv sync --frozen).
4. Keep scrape()'s return shape stable if you can. If you need to return PDFs/links separately, update ingest_url in src/ingestion/services/ingestion_service.py to match (both files are in scope for this ticket).

Files

• src/ingestion/scrapers/html_scraper.py — the extraction logic (main change).
• src/ingestion/services/ingestion_service.py — only if the scraper's output shape changes, or if you store PDFs/links as pointer entries (it currently hardcodes source_type="html").

🔬 Test targets (good representative pages)

• An SCS page with end-of-page accordion style FAQs: https://carleton.ca/scs/current-students/bachelor-of-cybersecurity/bcyber-courses-and-registration/
• A university FAQ page with a different design: https://carleton.ca/registration/new-ug/new-student-faqs/
• Any page containing PDF links and links to other pages.

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🧪 Tests

Save a few real-page HTML snapshots as fixtures under tests/ingestion/ and test the scraper against them offline (no network, no Docker). Assert that:

• accordion question + answer text now appears in the output,
• nav/header/footer boilerplate is gone,
• PDF links show up as [PDF: ...] markers.

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✅ Acceptance Criteria

• For pages with accordion/FAQ content, both the question and answer text appear in the scraped output (both failure modes fixed), and they stay together.
• Extracted content excludes nav/header/footer/sidebar boilerplate — main content only.
• PDF links are captured as [PDF: <url>] markers (and/or stored via the pointer source type); other useful links are preserved. Links/PDFs are not fetched, crawled, or downloaded.
• Chunking config and section_heading behavior are unchanged.
• Tests run against saved HTML fixtures (no network/Docker) and assert: recovered accordion Q&A, boilerplate removed, PDF markers present.
• make test and make lint pass. If beautifulsoup4 was added, the updated uv.lock is committed.