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-node-panel.tsx

@@ -19,6 +19,14 @@ const TYPE_LABELS: Record<string, string> = {
   halacha: "הלכה",
   topic: "נושא",
   practice_area: "תחום",
+  gap: "פסיקה חסרה",
+};
+
+const GAP_STATUS_LABELS: Record<string, string> = {
+  open: "ממתינה לקליטה",
+  uploaded: "הועלתה",
+  closed: "טופלה",
+  irrelevant: "לא רלוונטית",
 };
 
 const PA_LABELS: Record<string, string> = {
@@ -43,6 +51,7 @@ export function GraphNodePanel({
   onClose: () => void;
 }) {
   const isPrecedentLike = node.type === "precedent" || node.type === "halacha";
+  const isGap = node.type === "gap";
   return (
     <Card className="bg-surface border-rule shadow-sm w-80 shrink-0 overflow-y-auto">
       <CardContent className="space-y-4 p-4">
@@ -75,7 +84,21 @@ export function GraphNodePanel({
             <Row label="מקור" value={SOURCE_LABELS[node.source_kind] ?? node.source_kind} />
           )}
           {node.precedent_level && <Row label="דרגה" value={node.precedent_level} />}
-          {!isPrecedentLike && (
+          {isGap && (
+            <>
+              <Row label="מצוטטת ע״י" value={`${node.size} פסיקות בקורפוס`} />
+              {node.gap_status && (
+                <Row
+                  label="סטטוס"
+                  value={GAP_STATUS_LABELS[node.gap_status] ?? node.gap_status}
+                />
+              )}
+              <p className="text-ink-muted text-xs leading-relaxed m-0">
+                פסיקה זו מצוטטת בקורפוס אך אינה קיימת בו — מועמדת לקליטה.
+              </p>
+            </>
+          )}
+          {!isPrecedentLike && !isGap && (
             <p className="text-ink-muted text-xs leading-relaxed m-0">
               לחיצה על נקודה זו מתמקדת בשכניה — כל הפסיקות המשויכות אליה.
             </p>
@@ -90,6 +113,15 @@ export function GraphNodePanel({
             </Link>
           </Button>
         )}
+
+        {isGap && (
+          <Button asChild variant="outline" className="w-full">
+            <Link href="/missing-precedents">
+              <ExternalLink className="size-4 me-2" />
+              לרשימת הפסיקה החסרה
+            </Link>
+          </Button>
+        )}
       </CardContent>
     </Card>
   );