[Phase 4] Grounded teaching with citations UX

[ahmedfahim21]

Part of the Learnify 2.0 rebuild. Depends on #41, #45.

Context

Grounded sessions cite their sources. The Citations API is available on Bedrock: PDFs cite page numbers, custom-content chunks cite block indices, streamed via citations_delta. cited_text costs no output tokens. (Citations are incompatible with structured outputs — fine: widgets are emitted via tools, and concept-graph decomposition is a separate call.)

Tasks

• Grounded prompt assembly: source chunks as citation-enabled document blocks (citations: {enabled: true}) with cache_control — PDFs as base64 document blocks, URL/YouTube as custom-content blocks
• Handle citations_delta in the agent loop → citation markers streamed onto Narration
• Widget-level cite: {sourceId, quote} props validated server-side by substring match against source_chunks (drop invalid cites)
• Citation chips UI on Narration/ExplanationCard; click → source panel scrolls to the quoted chunk (PDF: page number, YouTube: timestamp)
• System-prompt grounding rules: when sources attached, factual claims must come from them
• Ungrounded topics keep working unchanged

Acceptance criteria

• Attach a YouTube lecture + a PDF → explanations carry citation chips; chip click jumps to the exact quote
• Detach sources → session runs ungrounded