fix(runtime): add app.storage module; make app a package; use relative import

app/storage.py

@@ -0,0 +1,32 @@
+# storage.py (no BOM)
+import os, tempfile
+from pathlib import Path
+
+def _first_writable(candidates):
+    for c in candidates:
+        if not c:
+            continue
+        try:
+            p = Path(c)
+            p.mkdir(parents=True, exist_ok=True)
+            t = p / ".write_test"
+            t.write_text("ok", encoding="utf-8")
+            try:
+                t.unlink()
+            except OSError:
+                pass
+            return p
+        except Exception:
+            continue
+    return Path(tempfile.mkdtemp(prefix="rag_"))
+
+DATA_DIR = _first_writable([
+    os.getenv("DATA_DIR") or None,
+    "/data",
+    "/app/data",
+    str(Path.home() / ".cache" / "rag_data"),
+    "/tmp/rag_data",
+])
+
+INDEX_DIR    = DATA_DIR / "index"
+HISTORY_JSON = DATA_DIR / "history.json"