feat(graph): research-gap (ghost) nodes (corpus graph PR C)

Turns the graph into a gap-finder: the 247 unresolved internal citations
(a corpus precedent cites a ruling NOT in the corpus) collapse to 230 distinct
"gap" nodes — each sized by how many corpus precedents cite it, i.e. the
most-wanted missing precedent.

Backend (web/graph_api.py — read-only, G2):
- "gap" added to VALID_NODE_TYPES (NOT default → off unless requested).
- New _gap_nodes_and_edges(): gap:<normalized citation> nodes from
  precedent_internal_citations WHERE cited_case_law_id IS NULL, sized by global
  in-degree; cites edges only from precedents present in the view (dangling-edge
  invariant holds). Best-effort enrichment from missing_precedents via exact
  normalized-citation match → gap_status + missing_precedent_id. Validated:
  230 gaps, top ע"א 3213/97 (cited 5×), 230/230 matched to missing_precedents.
- GraphNode += gap_status, missing_precedent_id. Metrics correctly exclude gap
  edges (target not a precedent). No app.py change (gated via node_types).

Frontend:
- graph.ts: GraphNodeType += "gap"; node fields.
- graph-filter-panel: toggle "חוסרי מחקר (פסיקה חסרה)" (off by default).
- graph-canvas: gaps render as faint hollow dashed circles, never recoloured
  by color-by; sized by citation count.
- graph-node-panel: gap branch — "מצוטטת ע״י N פסיקות" + status badge + link
  to /missing-precedents.

web-ui build + lint pass. Invariants: G2 (SELECT-only), UI2 (model grows on
explicit Pydantic). api:types post-deploy.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web-ui/src/components/graph/graph-view.tsx

@@ -65,6 +65,7 @@ export function GraphView() {
     showTopics: true,
     showPracticeAreas: true,
     showHalachot: false,
+    showGaps: false,
   });
   const facets = useGraphFacets().data;
   const [selectedNode, setSelectedNode] = useState<GraphNode | null>(null);
@@ -78,8 +79,14 @@ export function GraphView() {
     if (controls.showTopics) t.push("topic");
     if (controls.showPracticeAreas) t.push("practice_area");
     if (controls.showHalachot) t.push("halacha");
+    if (controls.showGaps) t.push("gap");
     return t.join(",");
-  }, [controls.showTopics, controls.showPracticeAreas, controls.showHalachot]);
+  }, [
+    controls.showTopics,
+    controls.showPracticeAreas,
+    controls.showHalachot,
+    controls.showGaps,
+  ]);
 
   // Metrics are needed when colouring by cluster or sizing by a centrality.
   const metricsOn =