Refactor and modularize fetchProblemsForSession() + integrate session performance summary

[smithrashell]

Problem

fetchProblemsForSession() currently has 5+ distinct responsibilities and is difficult to test or extend:

1. Pulls settings
2. Computes adaptive values
3. Fetches review problems
4. Fetches new problems
5. Deduplicates + pads session
6. Logs and returns session

This makes reasoning about the logic harder and prevents reusability in other flows (e.g. onboarding, review-only mode).

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Proposed Refactor

Break into 3 distinct layers:

• createSession() → High-level coordinator

• buildAdaptiveSessionSettings() → Handles adaptive sizing & difficulty logic

• fetchAndAssembleSessionProblems() → Handles fetching review + new problems, deduping, padding

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Labels

• refactor
• enhancement
• tech-debt
• priority: medium

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Outcome

• Cleaner, testable separation of concerns
• Prepares for session dashboards, mastery trends
• Lays groundwork for contributors or A/B testing in session logic