docs: add comprehensive architecture rules reference documentation

Created detailed documentation for all 51 architectural rules:

New Documentation:
- docs/development/architecture-rules.md (comprehensive reference)
- Added to mkdocs.yml navigation

Documentation Includes:
- Overview and usage instructions
- Severity level explanations
- All 51 rules organized by category:
  * Backend Rules (API, Service, Model, Exception)
  * Frontend Rules (JavaScript, Templates, Styling)
  * Naming Convention Rules
  * Security & Multi-Tenancy Rules
  * Code Quality Rules
  * Middleware Rules

For Each Rule:
- Rule ID and name
- Severity level
- Detailed description
- Good and bad code examples
- Anti-patterns detected
- File patterns affected

Additional Sections:
- Quick reference tables
- Pre-commit checklist
- Common violations and fixes
- Ignored patterns explanation
- Summary statistics (51 rules, 35 errors, 16 warnings)

This documentation complements:
- .architecture-rules.yaml (rule definitions)
- scripts/validate_architecture.py (enforcement)
- Code Quality guide
- Contributing guide

Documentation builds successfully with mkdocs.

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

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

models/database/base.py

@@ -2,6 +2,8 @@ from datetime import UTC, datetime
 
 from sqlalchemy import Column, DateTime
 
+from app.core.database import Base
+
 
 class TimestampMixin:
     """Mixin to add created_at and updated_at timestamps to models"""