feat: rewrite check-research for Gemini Deep Research (Task 5)

Replace all NotebookLM calls with Gemini Deep Research API
(pollResearch, parseResearchReport) and Gemini Infographics
(generateInfographicsForTopic with Imagen 4 Fast).

Changes:
- stepResearching: polls Gemini interaction instead of NotebookLM
- stepResearchComplete: generates infographics via Imagen (sync, no polling)
- stepInfographicsGenerating: legacy migration handler only
- Remove all NotebookLM imports and client initialization
- Add enableDeepResearch toggle (skip when disabled)
- Keep: fail-closed auth, stuck detection, enrichment, Claude critic

Co-authored-by: research <research@miriad.systems>

Author: Miriad