Correct two false doc comments (closes #403)

Two doc comments described behaviour that no longer exists. The comment on `ExportSnapshotMetadata` described an SQL dump uploaded under `snapshots/`; it now lists the steps the function performs and the two keys it writes. The comment on `EnsureStorageBinding` named `vaultik database purge`; the command is `vaultik database delete`. Comments only.

Model: opus-5-5

internal/snapshot/snapshot.go

@@ -271,16 +271,18 @@ func (sm *SnapshotManager) CompleteSnapshot(
 	return sm.MarkSnapshotComplete(ctx, snapshotID)
 }
 
-// ExportSnapshotMetadata exports snapshot metadata to S3
+// ExportSnapshotMetadata exports snapshot metadata to the backup destination store
 //
 // This method executes the complete snapshot metadata export process:
 // 1. Creates a temporary directory for working files
 // 2. Copies the main database to preserve its state
 // 3. Cleans the copy to contain only current snapshot data
-// 4. Dumps the cleaned database to SQL
-// 5. Compresses the SQL dump with zstd
-// 6. Encrypts the compressed data with age (always; recipients are required)
-// 7. Uploads to S3 at: snapshots/{snapshot-id}.sql.zst[.age]
+// 4. Vacuums the copy to remove the deleted data
+// 5. Compresses the copy with zstd
+// 6. Encrypts the compressed copy with age (always; recipients are required)
+// 7. Builds the blob manifest from the copy, compressed but not encrypted
+// 8. Uploads the database to metadata/{remote-key}/db.zst.age
+// 9. Uploads the manifest to metadata/{remote-key}/manifest.json.zst
 //
 // The only caller (finalizeSnapshotMetadata) does not close the main database
 // before calling this method: the index is copied at dbPath while it is still