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/development/troubleshooting.md

@@ -535,4 +535,4 @@ If the issue persists:
 
 - [PyCharm Make Configuration](pycharm-configuration-make.md) - Configure Make with virtual environment
 - [Contributing Guide](contributing.md) - Development workflow
-- [Database Migrations](database-migrations.md) - Working with Alembic
+- [Database Migrations](migration/database-migrations.md) - Working with Alembic