docs: add UserContext pattern documentation and architecture rules

Documentation:
- docs/architecture/user-context-pattern.md: Comprehensive guide on
  UserContext vs User model, JWT token mapping, common mistakes

Architecture Rules (auth.yaml):
- AUTH-005: Routes must use UserContext, not User model attributes
- AUTH-006: JWT token context fields must be defined in UserContext
- AUTH-007: Response models must match available UserContext data

Architecture Rules (module.yaml):
- MOD-024: Module static file mount order - specific paths first

These rules prevent issues like:
- Accessing SQLAlchemy relationships on Pydantic schemas
- Missing token fields causing fallback warnings
- Response model validation errors from missing timestamps
- 404 errors for module locale files due to mount order

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

mkdocs.yml

@@ -44,6 +44,7 @@ nav:
       - Observability: architecture/observability.md
       - Request Flow: architecture/request-flow.md
       - Authentication & RBAC: architecture/auth-rbac.md
+      - UserContext Pattern: architecture/user-context-pattern.md
       - Frontend Structure: architecture/frontend-structure.md
       - Models Structure: architecture/models-structure.md
       - Background Tasks: architecture/background-tasks.md