Split native Periscope (periscope/src) from inherited PinScope (periscope/dependency).

Keep validate.py and the finding engine as-is. AGPL LICENSE stays at the repo root.
Docker overlays dependency then src. Do not delete the inherited tree.

backend/__init__.py

@@ -0,0 +1,18 @@
+"""Backend package facade: native Periscope + inherited PinScope.
+
+``periscope/src/backend`` and ``periscope/dependency/backend`` are merged via
+pkgutil path extension. Do not put application modules in this directory.
+"""
+from __future__ import annotations
+
+import sys
+from pathlib import Path
+from pkgutil import extend_path
+
+_REPO = Path(__file__).resolve().parents[1]
+for _p in (_REPO / "periscope" / "src", _REPO / "periscope" / "dependency"):
+    _s = str(_p)
+    if _p.is_dir() and _s not in sys.path:
+        sys.path.insert(0, _s)
+
+__path__ = list(extend_path(__path__, __name__))