docs: reorganize migration docs into dedicated subfolder

- Create docs/development/migration/ directory
- Move database-migrations.md to migration subfolder
- Move svc-006-migration-plan.md to migration subfolder
- Update all cross-references in:
  - mkdocs.yml nav configuration
  - docs/index.md
  - docs/architecture/overview.md
  - docs/backend/overview.md
  - docs/development/contributing.md
  - docs/development/troubleshooting.md
  - docs/getting-started/database-setup.md

This separates migration plans from core documentation.

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

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

docs/architecture/overview.md

@@ -344,7 +344,7 @@ Internet ───────────│ Load Balancer│
 ### For Developers
 - [Creating a New Admin Page](../frontend/admin/page-templates.md)
 - [Backend Development Guide](../backend/overview.md)
-- [Database Migrations](../development/database-migrations.md)
+- [Database Migrations](../development/migration/database-migrations.md)
 
 ### For Operations
 - [Deployment Guide](../deployment/production.md)