feat: add 4 missing related_links tables to V3 migration (#52)

Add related_links collection tables for entities extending IdentifiedObject:
- customer_related_links for Customer entity (line 479-487)
- customer_account_related_links for CustomerAccount entity (line 599-607)
- electric_power_quality_summary_related_links for ElectricPowerQualitySummary (line 668-676)
- meter_related_links for Meter entity (line 795-803)

Each table follows existing pattern:
- FK constraint to parent entity with ON DELETE CASCADE
- Index on parent FK column for performance
- VARCHAR(1024) for related_links URLs

Part of ESPI 4.0 schema compliance remediation (Phase 1).
All entities extend IdentifiedObject and support Atom <link rel="related"> elements.

Migration tested successfully with H2 in-memory database.

Includes documentation:
- SCHEMA_COMPLIANCE_AUDIT_RESULTS.md: Comprehensive 11-phase remediation plan
- SCHEMA_COMPLIANCE_REMEDIATION_PLAN.md: Original analysis and plan
- FLYWAY_SCHEMA_SUMMARY.md: Complete Flyway table documentation
- MULTI_PHASE_PLAN_UPDATES.md: Phase planning notes

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

Co-authored-by: Claude Sonnet 4.5 <[email protected]>

Author: dfcoffin