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>

migrations/versions/011_add_pdfa_paths.py

@@ -0,0 +1,29 @@
+"""Add PDF/A archival variant path columns to files table
+
+Revision ID: 011_add_pdfa_paths
+Revises: 010_add_embedding_column
+Create Date: 2026-03-02
+
+"""
+
+from typing import Union
+
+import sqlalchemy as sa
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision: str = "011_add_pdfa_paths"
+down_revision: Union[str, None] = "010_add_embedding_column"
+depends_on: Union[str, None] = None
+
+
+def upgrade() -> None:
+    """Add PDF/A variant path columns to files table."""
+    op.add_column("files", sa.Column("original_pdfa_path", sa.String(), nullable=True))
+    op.add_column("files", sa.Column("processed_pdfa_path", sa.String(), nullable=True))
+
+
+def downgrade() -> None:
+    """Remove PDF/A variant path columns from files table."""
+    op.drop_column("files", "processed_pdfa_path")
+    op.drop_column("files", "original_pdfa_path")