feat: add inline document citation previews in answers

[Praneeth2711]

Summary

This PR adds inline document citation previews to answer sources.

Previously, document-backed answers displayed only source filenames. With this change, users can hover a source badge to view a preview of the retrieved document chunk that was used during answer generation.

Changes

Backend

• Added structured source metadata containing:

  • source filename
  • chunk index
  • preview text

• Added SourceChunk schema

• Added backward compatibility for legacy string-based sources

• Added citation utility helpers

Frontend

• Preserved existing source badge UI
• Added hover tooltip previews for source citations
• Supports both legacy and structured source formats

Compatibility

• No database migration required
• Existing sessions continue to load correctly
• Legacy string sources are automatically normalized

Tests

• Added citation-focused test suite
• Added backward compatibility coverage
• Verified database round-trip behavior
• Existing tests continue to pass

Notes

• Citation previews are hover-based in this initial implementation.
• Historical sessions remain fully supported.

Verification

• Uploaded document
• Generated document-backed answer
• Verified source badges render
• Verified citation preview tooltip displays retrieved chunk text
• Verified historical sessions continue to load correctly

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