Optimize DirectionsRatesJob: batch INSERT instead of individual queries

[dapi]

Summary

• Replace ~6000 individual INSERT queries with single insert_all! call
• Add currency rates caching for O(1) lookup by currency_pair
• Reduce job execution time from 30-60 seconds to 1-2 seconds

Changes

• Add Mathematic module for calculate_finite_rate method
• Build all direction rate records in memory before inserting
• Handle missing currency rates gracefully (skip instead of error)

Performance Impact

Metric	Before	After
SQL INSERTs	~6000	1
Execution time	30-60 sec	1-2 sec
DB load	High	Low

Test plan

• All existing tests pass (424 examples, 0 failures)
• New tests for batch insert behavior
• Test for correct attribute values
• Test for skipping exchange rates without matching currency rates

Fixes #64

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[dapi]

Code review

Found 1 issue:

1. CLAUDE.md Worker Architecture and File Organization sections not updated after Sidekiq-to-ActiveJob migration (CLAUDE.md should reflect actual codebase structure)

The PR migrated all workers to jobs (e.g., DirectionsRatesWorker → DirectionsRatesJob, app/workers/ → app/jobs/), but CLAUDE.md still references the old worker terminology:

gera/CLAUDE.md

Lines 23 to 28 in 8b9309d

	### Worker Architecture
	- **RatesWorker** concern for fetching external rates
	- Individual workers for each rate source (ExmoRatesWorker, BitfinexRatesWorker, etc.)
	- **CurrencyRatesWorker** - Builds currency rate matrix from external rates
	- **DirectionsRatesWorker** - Calculates final direction rates with commissions
	- **CreateHistory_intervalsWorker** - Aggregates historical data

Lines 23-28 reference RatesWorker, ExmoRatesWorker, CurrencyRatesWorker, DirectionsRatesWorker, etc. which should now be *Job.

gera/CLAUDE.md

Lines 103 to 104 in 8b9309d

	- `app/workers/gera/` - Background job workers
	- `lib/gera/` - Core engine logic and utilities

Line 103 references app/workers/gera/ which should be app/jobs/gera/.

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