PR-D (N-004): parsed content chunks viewer

[suguanYang]

Summary

• Add a workspace-scoped chunks API route for selected ready sources
• Map Knowhere document chunks to ParsedChunkView without persisting chunk bodies locally
• Load parsed sections into the center panel when a ready source is selected

Verification

• TDD red/green: pnpm test src/lib/chunks.test.ts failed before src/lib/chunks.ts, then passed
• pnpm lint
• pnpm test
• pnpm test:integration
• DATABASE_DRIVER=pg DATABASE_URL=... KNOWHERE_BASE_URL=https://api-staging.knowhereto.ai pnpm build
• Chrome MCP staging smoke: upload TXT source, wait for Processed · 1 sections, click source, parsed-content panel loads the section text from Knowhere chunks API
• Cleanup: local smoke row soft-deleted, temp fixture removed, staging document archived

Scope

N-004 only. Chat/retrieval/citation mapping stays for PR-E.