Add deterministic deduplication, rclone backend, and database purge command

- Implement deterministic blob hashing using double SHA256 of uncompressed
  plaintext data, enabling deduplication even after local DB is cleared
- Add Stat() check before blob upload to skip existing blobs in storage
- Add rclone storage backend for additional remote storage options
- Add 'vaultik database purge' command to erase local state DB
- Add 'vaultik remote check' command to verify remote connectivity
- Show configured snapshots in 'vaultik snapshot list' output
- Skip macOS resource fork files (._*) when listing remote snapshots
- Use multi-threaded zstd compression (CPUs - 2 threads)
- Add writer tests for double hashing behavior

Makefile

@@ -64,3 +64,6 @@ test-integration:
 local:
 	VAULTIK_CONFIG=$(HOME)/etc/vaultik/config.yml ./vaultik snapshot --debug list 2>&1
 	VAULTIK_CONFIG=$(HOME)/etc/vaultik/config.yml ./vaultik snapshot --debug create 2>&1
+
+install: vaultik
+	cp ./vaultik $(HOME)/bin/