fix(core): resolve all 12 adversarial review findings

- Zeroize passwords in CLI prompts, handlers, and mount authentication
- Preserve carrier_node_id when recovering Slot 0 via recovery key
- Add --slot parameter to restore-header for targeted slot recovery
- Implement online_backup and restore_from_backup using SQLite Online Backup API
- Expose 'sanctum backup' and 'sanctum restore' CLI subcommands
- Fix inactivity auto-lock by removing touch() from PROPFIND metadata/read_dir
- Support O_APPEND by setting file cursor to file size on handle creation
- Prevent data loss by implementing Drop for CarrierFile to flush dirty blocks
- Optimize CSPRNG padding to only fill unwritten slack space
- Batch carrier initialization in 500-block transactions to prevent UI/CLI freeze
- Enforce 64-byte savings threshold for LZ4 compression
- Add WebClient service diagnostic hint for Windows net use mount errors
- Add unit and integration tests covering all new features

src/crypto.rs

@@ -332,8 +332,8 @@ pub fn encrypt_chunk(
             vec![COMPRESSION_NONE]
         } else {
             let compressed = lz4_flex::compress_prepend_size(plaintext);
-            // Nur komprimieren, wenn tatsächlich Bytes gespart werden (+1 Byte für das Flag)
-            if compressed.len() + 1 < plaintext.len() {
+            // Nur komprimieren, wenn mindestens 64 Bytes eingespart werden (+1 Byte für das Flag)
+            if compressed.len() + 64 <= plaintext.len() {
                 let mut buf = Vec::with_capacity(compressed.len() + 1);
                 buf.push(COMPRESSION_LZ4);
                 buf.extend_from_slice(&compressed);
@@ -493,6 +493,22 @@ mod tests {
         assert_eq!(decrypted, plaintext);
     }
 
+    #[test]
+    fn test_lz4_chunk_compression_threshold() {
+        let dek = generate_dek();
+        // Unkomprimierbare Zufallsdaten (keine 64 Bytes Ersparnis)
+        let mut random_bytes = vec![0u8; 1000];
+        OsRng.fill_bytes(&mut random_bytes);
+
+        let (ct, nonce, tag) =
+            encrypt_chunk(&dek, 1, 0, &random_bytes, FORMAT_VERSION_V2).unwrap();
+        // Da Kompression keine 64 Bytes spart, wird COMPRESSION_NONE (1 Byte) + Plaintext gespeichert
+        assert_eq!(ct.len(), random_bytes.len() + 1);
+
+        let decrypted = decrypt_chunk(&dek, 1, 0, &ct, &nonce, &tag, FORMAT_VERSION_V2).unwrap();
+        assert_eq!(decrypted, random_bytes);
+    }
+
     #[test]
     fn test_v1_backward_compatibility() {
         let dek = generate_dek();