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/lib/api/graph.ts

@@ -14,7 +14,12 @@
 import { keepPreviousData, useQuery } from "@tanstack/react-query";
 import { apiRequest } from "./client";
 
-export type GraphNodeType = "precedent" | "halacha" | "topic" | "practice_area";
+export type GraphNodeType =
+  | "precedent"
+  | "halacha"
+  | "topic"
+  | "practice_area"
+  | "gap";
 
 export type GraphEdgeType =
   | "cites"
@@ -38,6 +43,8 @@ export type GraphNode = {
   pagerank: number | null; // normalized 0–1, only when metrics requested
   betweenness: number | null; // normalized 0–1
   community: number | null; // dense cluster id, 0 = largest
+  gap_status: string | null; // gap nodes only — open|uploaded|closed|irrelevant
+  missing_precedent_id: string | null; // gap nodes only
 };
 
 export type GraphFacets = {