fix: verify blob hash after download and decryption (closes #5) (#39)

## Summary

Add double-SHA-256 hash verification of decrypted plaintext in `FetchAndDecryptBlob`. This ensures blob integrity during restore operations by comparing the computed hash against the expected blob hash before returning data to the caller.

The blob hash is `SHA256(SHA256(plaintext))` as produced by `blobgen.Writer.Sum256()`. Verification happens after decryption and decompression but before the data is used.

## Test

Added `blob_fetch_hash_test.go` with tests for:
- Correct hash passes verification
- Mismatched hash returns descriptive error

## make test output

```
golangci-lint run
0 issues.

ok  git.eeqj.de/sneak/vaultik/internal/blob       4.563s
ok  git.eeqj.de/sneak/vaultik/internal/blobgen    3.981s
ok  git.eeqj.de/sneak/vaultik/internal/chunker    4.127s
ok  git.eeqj.de/sneak/vaultik/internal/cli        1.499s
ok  git.eeqj.de/sneak/vaultik/internal/config     1.905s
ok  git.eeqj.de/sneak/vaultik/internal/crypto     0.519s
ok  git.eeqj.de/sneak/vaultik/internal/database   4.590s
ok  git.eeqj.de/sneak/vaultik/internal/globals    0.650s
ok  git.eeqj.de/sneak/vaultik/internal/models     0.779s
ok  git.eeqj.de/sneak/vaultik/internal/pidlock    2.945s
ok  git.eeqj.de/sneak/vaultik/internal/s3         3.286s
ok  git.eeqj.de/sneak/vaultik/internal/snapshot   3.979s
ok  git.eeqj.de/sneak/vaultik/internal/vaultik    4.418s
```

All tests pass, 0 lint issues.

Co-authored-by: user <user@Mac.lan guest wan>
Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Reviewed-on: #39
Co-authored-by: clawbot <sneak+clawbot@sneak.cloud>
Co-committed-by: clawbot <sneak+clawbot@sneak.cloud>

internal/vaultik/restore.go

@@ -522,11 +522,23 @@ func (v *Vaultik) restoreRegularFile(
 
 // downloadBlob downloads and decrypts a blob
 func (v *Vaultik) downloadBlob(ctx context.Context, blobHash string, expectedSize int64, identity age.Identity) ([]byte, error) {
-	result, err := v.FetchAndDecryptBlob(ctx, blobHash, expectedSize, identity)
+	rc, err := v.FetchAndDecryptBlob(ctx, blobHash, expectedSize, identity)
 	if err != nil {
 		return nil, err
 	}
-	return result.Data, nil
+
+	data, err := io.ReadAll(rc)
+	if err != nil {
+		_ = rc.Close()
+		return nil, fmt.Errorf("reading blob data: %w", err)
+	}
+
+	// Close triggers hash verification
+	if err := rc.Close(); err != nil {
+		return nil, err
+	}
+
+	return data, nil
 }
 
 // verifyRestoredFiles verifies that all restored files match their expected chunk hashes