feat: AI Learning Assistant with DAG Roadmap Flow, Semantic Search, and Gemini LLM

[ankur-jat0009]

🎯 Overview

This PR implements the complete AI-Powered Learning Assistant for LearnSphere, fulfilling all assignment requirements:

• Learning Content Dataset: 8 comprehensive technical tracks (JavaScript, React, Node.js, Python, Java, SQL, Machine Learning, Full Stack) stored in structured SQLite with full schemas, indices, and modules.
• AI Recommendation Engine: Semantic TF-IDF scoring combined with live Google Gemini LLM, extracting learner skill level, timeframe, weekly hours, prerequisite gaps, and strategic rationales.
• DAG Roadmap Flowchart UI: Visual Directed Acyclic Graph (DAG) dependency nodes (\Foundational Gateway\ ➔ \Core Architecture\ ➔ \Enterprise Framework\ ➔ \Capstone Milestone) with interactive completion tracking, unlocks, project deliverables, and progress tracking.
• Structured Rationale Breakdown: 4 clear takeaway cards (Target Role Alignment, Pacing & Feasibility, Prerequisite Bridge, Portfolio Deliverable) replacing long walls of text.
• Immediate Next Step Card: Concrete first action, course module, duration, and milestone code deliverable.
• Conversational Tutor & History: Follow-up chat endpoint (/api/ai/chat), content catalog explorer, and query history persistence.
• Testing & Quality: 51/51 Vitest frontend tests pass, 11/11 Pytest backend tests pass, and Vite builds with 0 errors.

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