feat(graph): daily-digest (יומון) discovery layer (corpus graph PR E)

Chaim's idea: surface the downloaded "כל יום" digests in the graph. Each digest
COVERS the ruling it analyses — a corpus precedent when we have it (16), or a
synthesized gap node from its underlying_citation when we don't (269). So the
digest layer doubles as a discovery signal: it makes visible that the daily
feed overwhelmingly covers rulings NOT yet in the corpus.

Backend (web/graph_api.py — read-only, G2):
- "digest" added to VALID_NODE_TYPES (off by default).
- _digest_nodes_and_edges(): dig:<id> nodes from completed digests, `covers`
  edge → cl:precedent (linked_case_law_id in view) or → gap:<underlying_citation>
  (synthesized, deduped against the gap layer — real in-degree wins). Carries
  concept_tag (label), headline_holding (note), underlying_court/date.
- _add_digests() appends the layer with gap dedup. Wired into both build
  functions. GraphNode += note, digest_id. Gated via node_types (no app.py
  change). Validated: 16 covers→precedent, 269 covers→gap.

Frontend:
- graph.ts: GraphNodeType += "digest"; GraphEdgeType += "covers"; node fields.
- graph-filter-panel: toggle "יומונים (כל יום)" (off by default).
- graph-canvas: digest = teal node (r=4); `covers` edges teal.
- graph-node-panel: digest branch — concept + holding + court/date + link to
  /digests.

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

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

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

@@ -66,6 +66,7 @@ export function GraphView() {
     showPracticeAreas: true,
     showHalachot: false,
     showGaps: false,
+    showDigests: false,
   });
   const facets = useGraphFacets().data;
   const [selectedNode, setSelectedNode] = useState<GraphNode | null>(null);
@@ -80,12 +81,14 @@ export function GraphView() {
     if (controls.showPracticeAreas) t.push("practice_area");
     if (controls.showHalachot) t.push("halacha");
     if (controls.showGaps) t.push("gap");
+    if (controls.showDigests) t.push("digest");
     return t.join(",");
   }, [
     controls.showTopics,
     controls.showPracticeAreas,
     controls.showHalachot,
     controls.showGaps,
+    controls.showDigests,
   ]);
 
   // Metrics are needed when colouring by cluster or sizing by a centrality.