Harden image fetch algorithm

Prepare for database integration

modules/shared/SetsDict.py

@@ -0,0 +1,25 @@
+class SetsDict:
+    def __init__(self):
+        self._data: dict[str, set] = dict()
+
+    def add(self, key, value):
+        if key not in self._data: self._data[key] = set()
+        if value not in self._data[key]: self._data[key].add(value)
+
+    def delete(self, key, value):
+        if self._data.get(key, None): self._data[key].remove(value)
+
+    @property
+    def index(self):
+        res = set()
+        for key, collection in self._data.items():
+            for elem in collection:
+                if elem not in res:
+                    res.add(elem)
+        return res
+
+    def by_key(self, key):
+        return self._data.get(key, None)
+
+    @property
+    def keys(self): return self._data.keys()