feat(graph): duplicate a selected subgraph server-side

A secret-safe equivalent of blueprint import: the graph editor's overflow menu
gains "Duplicate selection", which deep-clones the selected value and
colour-strip sources server-side (full config preserved, never crossing the
wire) and rewires references that point within the selection — shared
dependencies (devices, HA sources, …) stay shared.

- graph_schema.remap_refs: write-twin of extract_refs (same dot/list/bindable
  grammar) that rewrites only in-selection ids; 8 unit tests.
- BaseSqliteStore.clone(): faithful deep-copy clone (no schema round-trip, so no
  field is lost), prefix-preserving fresh id; reusable by any store.
- POST /api/v1/graph/duplicate: two-pass clone-then-rewire restricted to value /
  colour-strip sources (no inline secrets), with a safety net flagging any
  unremapped reference; 7 integration tests vs real stores.
- Frontend: duplicateSubgraph (+cache invalidation), graphDuplicateSelection
  (reload + reselect the new cluster), overflow-menu item, i18n (en/ru/zh).

server/src/ledgrab/api/graph_schema.py

@@ -279,6 +279,54 @@ def extract_refs(entity: dict[str, Any], field_path: str) -> list[str]:
     return [v for v in current if isinstance(v, str) and v]
 
 
+def remap_refs(entity: dict[str, Any], field_path: str, id_map: dict[str, str]) -> int:
+    """Rewrite referenced ids under ``field_path`` *in place*, using ``id_map``.
+
+    The write-twin of :func:`extract_refs`: it walks the same dot/list/bindable
+    grammar and replaces any leaf id present in ``id_map`` with its mapped value.
+    Ids absent from ``id_map`` (references to entities outside the remap set) are
+    left untouched, so a clone keeps sharing its un-cloned dependencies. Unbound
+    bindables (a plain number where an object was expected) and missing keys are
+    tolerated. Returns the number of ids rewritten.
+    """
+    segments = field_path.split(".")
+    # Descend to the container(s) that hold the final key.
+    parents: list[Any] = [entity]
+    for segment in segments[:-1]:
+        is_list = segment.endswith("[]")
+        key = segment[:-2] if is_list else segment
+        nxt: list[Any] = []
+        for obj in parents:
+            if not isinstance(obj, dict):
+                continue
+            val = obj.get(key)
+            if is_list:
+                if isinstance(val, list):
+                    nxt.extend(val)
+            elif isinstance(val, dict):
+                nxt.append(val)
+        parents = nxt
+
+    last = segments[-1]
+    last_is_list = last.endswith("[]")
+    key = last[:-2] if last_is_list else last
+    count = 0
+    for obj in parents:
+        if not isinstance(obj, dict):
+            continue
+        val = obj.get(key)
+        if last_is_list:
+            if isinstance(val, list):
+                for i, item in enumerate(val):
+                    if isinstance(item, str) and item in id_map:
+                        val[i] = id_map[item]
+                        count += 1
+        elif isinstance(val, str) and val in id_map:
+            obj[key] = id_map[val]
+            count += 1
+    return count
+
+
 def serialize_entity(model: Any) -> dict[str, Any]:
     """Best-effort serialize a storage model to a plain dict for graph use.