feat(pdfa): add FreeTSA timestamping, per-provider folder overrides, individual upload toggles

- Add RFC 3161 timestamping via FreeTSA (PDFA_TIMESTAMP_ENABLED, PDFA_TIMESTAMP_URL)
- Replace PDFA_UPLOAD_TO_PROVIDERS with individual PDFA_UPLOAD_ORIGINAL and PDFA_UPLOAD_PROCESSED
- Add PDFA_UPLOAD_FOLDER setting for per-provider subfolder configuration
- Add GOOGLE_DRIVE_PDFA_FOLDER_ID for Google Drive-specific folder override
- Add folder_override parameter to all 8 folder-using upload tasks
- Add folder_overrides dict parameter to send_to_all_destinations
- Add _compute_pdfa_folder_overrides() and _timestamp_file() helpers
- Expand tests to 26 (timestamping, folder overrides, individual toggles)
- Update docs/ConfigurationGuide.md and .env.demo

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

.env.demo

@@ -356,6 +356,17 @@ NEAR_DUPLICATE_THRESHOLD=0.85
 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
+# Upload original-file PDF/A variant to all configured storage providers
+PDFA_UPLOAD_ORIGINAL=false
+# Upload processed-file PDF/A variant to all configured storage providers
+PDFA_UPLOAD_PROCESSED=false
+# Subfolder name appended to each provider's folder for PDF/A uploads
+# e.g. if Dropbox folder is '/Documents' this puts PDF/A files into '/Documents/pdfa'
+PDFA_UPLOAD_FOLDER=pdfa
+# Google Drive folder ID for PDF/A uploads (uses folder IDs, not paths)
+# Leave empty to use the same folder as regular uploads
+GOOGLE_DRIVE_PDFA_FOLDER_ID=
+# RFC 3161 timestamping of PDF/A files (creates .tsr proof-of-existence files)
+PDFA_TIMESTAMP_ENABLED=false
+# Timestamp Authority URL (default: FreeTSA, a free RFC 3161 TSA)
+PDFA_TIMESTAMP_URL=https://freetsa.org/tsr