[Insight extractor] Feature: Rich Hover Preview for Source References

### ✨ Feature: Rich Hover Preview for Source References

**User Story**
As a user, when I see a numbered reference in the chat (e.g., “²”), I want to hover over it to see more context about the source (page number, snippet, relevance) before I click, so I can quickly judge whether it’s worth opening.

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### 📝 Current Behavior
- References appear as small numbered badges (e.g., ¹, ²).
- Clicking opens the linked PDF page + chunk.
- Hover only shows “Page X, Chunk Y” (not very helpful).

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### 🎯 Expected Behavior
- Hovering over a reference badge shows a **tooltip/card preview** with:
- Document title (truncated if long).
- Page number + chunk.
- Text snippet from the source (first 2–3 lines).
- (Optional) Relevance note if available.
- Clicking still navigates to the correct PDF page + highlighted span.

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### ✅ Acceptance Criteria
- [ ] Hovering over numbered reference shows expanded preview.
- [ ] Preview displays document title, snippet, and page/chunk info.
- [ ] Preview is styled consistently (not blocking main text).
- [ ] Clicking reference still navigates to PDF view at correct location.
- [ ] Works across all references in chat responses.

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### 🔨 Task Breakdown
- [ ] **Backend**
- Ensure each reference includes snippet + metadata (document title, page, chunk).

- [ ] **Frontend**
- Implement hover tooltip/card component.
- Populate with snippet + metadata.
- Ensure tooltip is responsive and doesn’t overflow small screens.