Add relationship detection and display support

[glennswest]

Summary

• Add support for detecting and displaying SQLAlchemy relationships in the admin interface
• New relationships.py module that detects BelongsTo, HasOne, HasMany, and ManyToMany relationships
• Auto-detects suitable display fields for related models (name, title, hostname, etc.)

Changes

• New file: admin_interface/relationships.py - Core relationship detection logic
• New templates:
  • related_link.html - Link to related model
  • related_single.html - Display single related record
  • related_table.html - Display multiple related records
• Updated: admin_site.py, model_view.py - Integration with relationship detection
• Updated: create.html, update.html, list_content.html - Display related data

Features

• Automatically detects relationships defined on SQLAlchemy models
• Loads and displays related data in create/update forms
• Shows relationship summaries in list views
• Supports foreign key relationships in both directions

Test plan

• Test with models that have foreign key relationships
• Verify BelongsTo relationships show parent record info
• Verify HasMany relationships show child record counts
• Test create/update forms with relationship fields