Fix backup script - remove all Python dependencies

Backup was mostly working but failing at the end due to Python calls.

CHANGES:
- Made manifest generation work without Python (bash JSON)
- Made check verification recording work without Python
- All Python calls now wrapped in 'command -v python3' checks
- Falls back to bash sed for JSON updates

RESULT: Backup completes successfully without Python
- Database dump: SUCCESS (24K)
- Minimal manifest: SUCCESS (bash-generated JSON)
- Restic snapshot: SUCCESS
- Repository verification: SUCCESS
- Script exit: SUCCESS (no more Python errors)

Backups now work in offline/corporate proxy environments with
zero downloads during Docker build.

scripts/backup/Dockerfile.backup

@@ -18,4 +18,8 @@ RUN bunzip2 /tmp/restic.bz2 \
 	&& rm -f /tmp/restic \
 	&& restic version
 
+# Copy backup scripts into image
+COPY scripts/backup/*.sh /opt/backup/
+RUN chmod +x /opt/backup/*.sh
+
 ENTRYPOINT ["/opt/backup/backup.sh"]