fix(backup): resolve relative SSTable paths in incremental backup

MANIFEST stores relative paths like 'sstables/1.sst'.
The old code checked fileExists against CWD instead of dataDir,
silently omitting SSTables when run from another directory.
Now resolves paths relative to dataDir.

src/barabadb/core/backup.nim

@@ -407,11 +407,12 @@ proc incrementalBackupDataDir*(dataDir: string, output: string, verbose: bool =
     let manifest = parseJson(readFile(manifestPath))
     for node in manifest{"sstables"}:
       let sstPath = node{"path"}.getStr()
-      if fileExists(sstPath):
-        filesToInclude.add(sstPath)
+      let absSstPath = if isAbsolute(sstPath): sstPath else: dataDir / sstPath
+      if fileExists(absSstPath):
+        filesToInclude.add(absSstPath)
       else:
         if verbose:
-          echo "WARNING: SSTable missing: ", sstPath
+          echo "WARNING: SSTable missing: ", absSstPath
   except CatchableError as e:
     echo "ERROR: Failed to parse MANIFEST: ", e.msg
     return false