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

TODO.md

@@ -2,6 +2,37 @@
 
 Linear list of tasks to complete before 1.0 release.
 
+## Rclone Storage Backend (Complete)
+
+Add rclone as a storage backend via Go library import, allowing vaultik to use any of rclone's 70+ supported cloud storage providers.
+
+**Configuration:**
+```yaml
+storage_url: "rclone://myremote/path/to/backups"
+```
+User must have rclone configured separately (via `rclone config`).
+
+**Implementation Steps:**
+1. [x] Add rclone dependency to go.mod
+2. [x] Create `internal/storage/rclone.go` implementing `Storer` interface
+   - `NewRcloneStorer(remote, path)` - init with `configfile.Install()` and `fs.NewFs()`
+   - `Put` / `PutWithProgress` - use `operations.Rcat()`
+   - `Get` - use `fs.NewObject()` then `obj.Open()`
+   - `Stat` - use `fs.NewObject()` for size/metadata
+   - `Delete` - use `obj.Remove()`
+   - `List` / `ListStream` - use `operations.ListFn()`
+   - `Info` - return remote name
+3. [x] Update `internal/storage/url.go` - parse `rclone://remote/path` URLs
+4. [x] Update `internal/storage/module.go` - add rclone case to `storerFromURL()`
+5. [x] Test with real rclone remote
+
+**Error Mapping:**
+- `fs.ErrorObjectNotFound` → `ErrNotFound`
+- `fs.ErrorDirNotFound` → `ErrNotFound`
+- `fs.ErrorNotFoundInConfigFile` → `ErrRemoteNotFound` (new)
+
+---
+
 ## CLI Polish (Priority)
 
 1. Improve error messages throughout