Add PDF text extraction workflow

Request:
- Provide a way to install or develop a PDF extraction tool for archived HK IPO documents.

Changes:
- Add requirements.txt with pypdf as the lightweight PDF text extraction dependency.
- Add scripts/extract_pdf_text.py to extract text from PDF source_refs into repo-relative data/extracted_text files.
- Add extracted text outputs and an extracted_text_manifest snapshot for the six archived HKEXnews PDFs.
- Document the extraction workflow in README.md.
- Ignore .venv and keep generated SQLite/Python transient files out of git.
- Use extracted text to verify the 06106 full prospectus, update source_refs, remove the related data gap, and fill 06106 offering terms.

Verification:
- Installed python3.14-venv system support, created a local .venv, and installed requirements.txt.
- Re-ran scripts/bootstrap_historical_data.py and scripts/extract_pdf_text.py.
- Verified extracted text paths and hashes against data/snapshots/extracted_text_manifest.csv.
- Verified SQLite integrity and snapshot row counts.
- Ran git diff --cached --check and searched durable files for machine-specific absolute paths.

requirements.txt

@@ -0,0 +1 @@
+pypdf>=6.1,<7