refactor: Address Copilot code review recommendations

Address 3 of 4 Copilot recommendations from PR review:

1. [Rejected] SustainabilityPageDataID [JsonIgnore] attribute
   - Intentionally excluded from JSON serialization
   - Internal DB identifier only used for backend query filtering
   - Frontend doesn't need this ID - proper encapsulation

2. [Fixed] Extract magic number 10 to named constant
   - Added MAX_TREND_REPORTS constant in TrendChart.tsx
   - Documents relationship to pagination limit
   - Improves maintainability

3. [Fixed] Add user-facing message for insufficient trend data
   - Previously returned null silently when < 2 reports
   - Now shows helpful message: "Trend analysis requires at least 2 data points"
   - Better UX for users with only 1 report

4. [Fixed] Remove unnecessary null-coalescing operator
   - LoadMoreHistory parameter is non-nullable
   - Removed redundant commandData?.PageIndex ?? 0
   - Simplified to commandData.PageIndex

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: liamgold