Bootstrap HK IPO historical archive

Request:
- Use the project archivist workflow to update historical IPO data.

Changes:
- Add an embedded SQLite archive at data/hk_ipo.sqlite.
- Add schema/hk_ipo.schema.sql and scripts/bootstrap_historical_data.py for reproducible archive generation.
- Archive HKEXnews source PDFs for 06658, 06675, and 06106 under repo-relative data/raw paths.
- Export Git-friendly snapshots for ipo_master, offering_terms, ipo_demand, source_refs, and data_gaps.
- Add .gitignore rules for Python cache and SQLite transient files.

Verification:
- Re-ran the bootstrap script successfully.
- Ran PRAGMA integrity_check on the SQLite database.
- Verified source_refs paths are repo-relative, files exist, and SHA-256 hashes match.
- Verified snapshot row counts match SQLite table counts.
- Ran git diff --check and searched generated durable files for machine-specific absolute paths.

.gitignore

@@ -0,0 +1,4 @@
+__pycache__/
+data/*.sqlite-journal
+data/*.sqlite-shm
+data/*.sqlite-wal