Add benchmarking project for performance testing

[ChuckNovice]

Summary

• Create RecurringThings.Benchmarks project using BenchmarkDotNet
• Add benchmark classes for Query, Create, Update, and Delete operations
• Use varied RRule patterns (DAILY, WEEKLY, MONTHLY, YEARLY) for realistic tests
• Include progress output during setup/cleanup phases to avoid blank screen
• Run migrations before benchmarks to exclude migration time from results
• Generate PNG charts via ScottPlotExporter
• Add benchmarking section to README.md

Benchmark Scenarios

Scenario	Description	Parameters
Query virtualized	Get 3 months of virtualized occurrences	100/1000/10000 recurrences
Query recurrences	Get recurrence patterns only	100/1000/10000 recurrences
Query with filter	Get occurrences filtered by type	100/1000/10000 recurrences
Create recurrence	Create new recurrence pattern	Both providers
Create occurrence	Create standalone occurrence	Both providers
Update standalone	Update standalone occurrence	100 occurrences
Update virtualized	Update virtualized (creates override)	100 occurrences
Delete occurrence	Delete standalone occurrence	Both providers
Delete recurrence	Cascade delete recurrence	Both providers

Test plan

• Build: dotnet build -c Release benchmarks/RecurringThings.Benchmarks
• All unit tests pass
• Code formatting verified
• Run benchmarks locally with MongoDB
• Run benchmarks locally with PostgreSQL
• Verify PNG charts generated in BenchmarkResults/

🤖 Generated with Claude Code