feat(graph): add scalable snapshot structure graph explorer (web + server)

[saurabhsharma2u]

Motivation

• Provide an interactive structure graph view for a snapshot that lets users pan/zoom, inspect URLs, and apply filters while supporting very large graphs (target: 10k nodes) without UI lag.
• Push heavy filtering/limits server-side to keep browser rendering responsive and avoid transferring huge payloads.

Description

• Add a server endpoint GET /api/graph/snapshot that returns a lightweight graph projection for a snapshot with server-side filters (maxNodes, maxEdges, minPageRank, minInlinks, minOutlinks, role, search) and truncation metadata; implemented in packages/server/src/index.ts.
• Introduce typed API contracts and a client helper fetchSnapshotGraph in packages/web/src/api.ts (SnapshotGraphNode, SnapshotGraphEdge, SnapshotGraphResponse, SnapshotGraphQuery).
• Implement a new high-performance canvas-based UI GraphPage in packages/web/src/pages/GraphPage.tsx with pan/zoom, hover tooltips, URL search and numeric filters, maxNodes control, and an edge-truncation notice for performance.
• Wire the page into the application router at /graph by updating packages/web/src/App.tsx and expose server-side SQL that aggregates in/out counts and limits node/edge results for predictable client-side rendering.

Testing

• Ran a production build for the web package with pnpm --filter @crawlith/web build, which completed successfully. ✅
• Type-check for the server with pnpm exec tsc -p packages/server/tsconfig.json --noEmit failed due to workspace type resolution in this environment (@crawlith/core types not resolved), not related to the introduced endpoint logic. ⚠️
• Attempted an automated browser capture via Playwright to validate the page render, but the Chromium instance crashed in this environment (TargetClosedError / SIGSEGV), so no screenshot artifact was produced. ⚠️

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Codex Task