feat(pdfa): add PDF/A archival conversion using ocrmypdf

- Add ENABLE_PDFA_CONVERSION, PDFA_FORMAT, PDFA_UPLOAD_TO_PROVIDERS config settings
- Add original_pdfa_path and processed_pdfa_path columns to FileRecord model
- Create Alembic migration 011_add_pdfa_paths
- Create app/tasks/convert_to_pdfa.py Celery task using ocrmypdf + Ghostscript
- Integrate PDF/A conversion into finalize_document_storage pipeline
- Add comprehensive unit tests (15 tests)
- Update .env.demo and docs/ConfigurationGuide.md

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

.env.demo

@@ -346,4 +346,16 @@ ENABLE_DEDUPLICATION=True
 SHOW_DEDUPLICATION_STEP=True
 # Minimum cosine similarity score (0–1) for two documents to be flagged as
 # near-duplicates.  0.85 means 85 % semantic overlap.  Lower = more matches.
-NEAR_DUPLICATE_THRESHOLD=0.85
+NEAR_DUPLICATE_THRESHOLD=0.85
+
+# **PDF/A Archival Conversion**
+# When enabled, PDF/A copies of both the original ingested file and the processed
+# file are created and saved alongside the standard copies.  This may double or
+# triple storage but provides better legal coverage with time-stamped archival copies.
+# Uses ocrmypdf with Ghostscript for the conversion.
+ENABLE_PDFA_CONVERSION=false
+# PDF/A format variant: 1 = PDF/A-1b, 2 = PDF/A-2b (default), 3 = PDF/A-3b
+PDFA_FORMAT=2
+# Also upload the processed PDF/A variant to all configured storage providers
+# (files are uploaded with a '-PDFA' suffix in their filename)
+PDFA_UPLOAD_TO_PROVIDERS=false