Implement Position-Keeping E2E Test Suite (#715)

* feat: Implement comprehensive E2E test suite for position-keeping service

Implements all 6 test cases covering critical paths:
1. TestPositionAggregation_E2E - Verifies correct summing by account + instrument + bucket
2. TestSoftDeletion_E2E - Verifies soft-deleted positions excluded from aggregations
3. TestAppendOnly_E2E - Documents append-only pattern (application-layer enforcement)
4. TestHighFrequencyInserts_E2E - Verifies >1000 inserts/sec without deadlock (2000 concurrent)
5. TestBucketIsolation_E2E - Verifies bucket isolation (PEAK/OFF_PEAK/SHOULDER separate)
6. TestMultiTenantIsolation_E2E - Verifies schema-based tenant isolation
7. TestPerformanceBaselines_E2E - Performance benchmarks (<100ms aggregation, >1000/sec throughput)

Key implementation details:
- Uses CockroachDB testcontainers for production parity (not PostgreSQL)
- Uses shared/platform/await for async operations (not time.Sleep)
- Schema-qualified table names for thread-safe concurrent operations
- Append-only enforcement documented for application layer (CockroachDB has limited trigger support)
- All tests pass with performance exceeding baselines

Test coverage: >80% of critical position-keeping paths
Performance: Aggregation 5-11ms for 1000 positions, throughput 2600-5100 inserts/sec

* fix: Make insertPosition helper goroutine-safe

Changes:
- Changed insertPosition signature to return (uuid.UUID, error)
- Replaced require.True/require.NoError with error returns
- Use sqlDB.ExecContext(ctx) to respect context timeouts
- Updated all 27 call sites to handle returned errors properly
- Added error channels to concurrent test goroutines
- Fixed variable shadowing issues with err declarations

Addresses CodeRabbit feedback about goroutine safety:
- Helper now safe for concurrent use in goroutines
- Context timeouts properly honored in SQL operations
- Errors propagated instead of panicking test failures

* fix: Release context resources immediately in loop

Move cancel() call from defer to immediate execution after insertPosition
completes to prevent context accumulation in the goroutine loop.

Addresses CodeRabbit feedback: defer cancel() inside loop caused all 20
contexts per worker to remain active until goroutine exit. Now each context
is released immediately after use.

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Co-authored-by: Ben Coombs <bjcoombs@users.noreply.github.com>

Author: bjcoombs