feat(analytics): generate detailed progress insights for students

[blessingernest54]

Closes #405

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Description

Generate detailed progress insights for students and instructors.

Changes

• types.rs: Added LearningVelocity, SkillGap, and ProgressInsightsReport structs
• lib.rs: Added generate_progress_insights — single call returns all four report sections
• tests.rs: 6 tests covering all sections and the StudentNotFound error case

Reports Covered

Report	Implementation
Learning velocity	sessions/day, modules/week, avg daily time, streak, trend
Peer comparison	percentile rank among enrolled students, score delta vs course avg
Skill gaps	modules with score < 70, severity = 70 − score, remediation hint
Next steps	adaptive recommendations based on score tier and streak status

Acceptance Criteria

• Reports generate <5s — single storage scan, O(sessions + peers)
• Insights actionable — each skill gap includes a remediation string; next steps include priority and adaptive flag
• Visualization clear — structured ProgressInsightsReport with typed fields ready for client rendering

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