Add snapshot graph API and cluster-first graph explorer UI

[saurabhsharma2u]

Motivation

• Provide a scalable, cluster-first graph projection for snapshots so the UI can visualize site structure at multiple levels without overwhelming the client.
• Allow on-demand edge rendering and 1-hop neighbor queries to keep initial graph payloads small and interactive.

Description

• Add server endpoints GET /api/graph/snapshot and GET /api/graph/neighbors that expose hierarchical node projections (sections → clusters → urls) and optional internal edges, with filtering options like level, minPageRank, minInlinks, minOutlinks, maxNodes, maxEdges, search, and includeEdges.
• Introduce lightweight SnapshotGraphNode and SnapshotGraphEdge payload shapes on the server and client, and return metadata about truncation and counts.
• Implement client-side API functions fetchSnapshotGraph and fetchGraphNeighbors and extend api.ts types to include SnapshotGraphResponse and query options.
• Add a new GraphPage component implementing a cluster-first radial layout with progressive LOD (three zoom levels), heat rings, interaction-driven neighbor/edge fetch, canvas rendering, and controls for filtering and node selection.
• Wire the new page into the router in App.tsx (route /graph).

Testing

• Ran a TypeScript build to validate types across packages/server and packages/web which completed successfully (yarn build / npm run build).
• Executed the web dev build and loaded the app locally to exercise the new /graph page and the new endpoints, confirming responses and canvas rendering (no automated unit tests were added for the new features).

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Codex Task