refactor: move fallback error templates to shared directory with improved naming

Reorganize error template structure to consolidate shared templates in a dedicated location.

Changes:
- Move templates/fallback/* to templates/shared/ with -fallback suffix
  - 404.html → 404-fallback.html
  - 500.html → 500-fallback.html
  - generic.html → generic-fallback.html
- Update error_renderer.py to use new template paths
  - Modified template selection logic in _find_template()
  - Updated documentation strings to reflect new paths
- Update error rendering documentation
  - HTML_ERROR_RENDERING_FLOW_DIAGRAM.md: Updated diagrams and examples
  - ERROR_RENDERING_DEVELOPER_DOCUMENTATION.md: Updated file structure and paths

Benefits:
- Clearer organization with all shared templates in shared/ directory
- Consistent naming convention (-fallback suffix) indicates purpose
- Aligns with existing cdn-fallback.html in same directory

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

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

app/exceptions/error_renderer.py

@@ -69,8 +69,8 @@ class ErrorPageRenderer:
         Template Selection Priority:
         1. Context-specific error page: {context}/errors/{status_code}.html
         2. Context-specific generic: {context}/errors/generic.html
-        3. Fallback error page: fallback/{status_code}.html
-        4. Fallback generic: fallback/generic.html
+        3. Shared fallback error page: shared/{status_code}-fallback.html
+        4. Shared fallback generic: shared/generic-fallback.html
 
         Args:
             request: FastAPI request object
@@ -150,8 +150,8 @@ class ErrorPageRenderer:
         Priority:
         1. {context}/errors/{status_code}.html
         2. {context}/errors/generic.html
-        3. fallback/{status_code}.html
-        4. fallback/generic.html
+        3. shared/{status_code}-fallback.html
+        4. shared/generic-fallback.html
         """
         templates_dir = ErrorPageRenderer.get_templates_dir()
 
@@ -176,13 +176,13 @@ class ErrorPageRenderer:
         if (templates_dir / generic_template).exists():
             return generic_template
 
-        # Try fallback status code template
-        fallback_specific = f"fallback/{status_code}.html"
+        # Try shared fallback status code template
+        fallback_specific = f"shared/{status_code}-fallback.html"
         if (templates_dir / fallback_specific).exists():
             return fallback_specific
 
-        # Use fallback generic template (must exist)
-        return "fallback/generic.html"
+        # Use shared fallback generic template (must exist)
+        return "shared/generic-fallback.html"
 
     @staticmethod
     def _prepare_template_data(