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-canvas.tsx

@@ -50,6 +50,7 @@ const NODE_COLORS: Record<string, string> = {
   halacha: "#b45309", // amber
   topic: "#a97d3a", // gold — hubs stand out
   practice_area: "#475569", // slate
+  gap: "#94a3b8", // faint slate — research gap (not in corpus)
 };
 
 const TREATMENT_COLORS: Record<string, string> = {
@@ -215,13 +216,23 @@ export function GraphCanvas({
     (node: FGNode, ctx: CanvasRenderingContext2D, globalScale: number) => {
       const r = radiusForNode(node, sizeBy);
       const dimmed = isDimmed(node.id);
+      const isGap = node.type === "gap";
       const color = colorForNode(node, colorBy);
-      ctx.globalAlpha = dimmed ? 0.18 : 1;
+      ctx.globalAlpha = dimmed ? 0.18 : isGap ? 0.55 : 1;
 
       ctx.beginPath();
       ctx.arc(node.x ?? 0, node.y ?? 0, r, 0, 2 * Math.PI);
-      ctx.fillStyle = color;
-      ctx.fill();
+      if (isGap) {
+        // Hollow dashed circle — a ruling cited but absent from the corpus.
+        ctx.setLineDash([3 / globalScale, 2 / globalScale]);
+        ctx.lineWidth = 1.3 / globalScale;
+        ctx.strokeStyle = NODE_COLORS.gap;
+        ctx.stroke();
+        ctx.setLineDash([]);
+      } else {
+        ctx.fillStyle = color;
+        ctx.fill();
+      }
       if (node.id === activeId) {
         ctx.lineWidth = 2 / globalScale;
         ctx.strokeStyle = "#a97d3a";