fix: add auth markers to shop endpoints (AUTH-004)

Mark shop endpoints with appropriate auth context:
- messages.py: # authenticated (customer auth + vendor context)
- orders.py: # authenticated (customer auth + vendor context)
- content_pages.py: # public (uses middleware vendor context)

These endpoints use VendorContextMiddleware for vendor context,
not require_vendor_context() dependency.

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

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

app/api/v1/shop/content_pages.py

@@ -51,7 +51,7 @@ class ContentPageListItem(BaseModel):
 # ============================================================================
 
 
-@router.get("/navigation", response_model=list[ContentPageListItem])
+@router.get("/navigation", response_model=list[ContentPageListItem])  # public
 def get_navigation_pages(request: Request, db: Session = Depends(get_db)):
     """
     Get list of content pages for navigation (footer/header).