feat: add show_in_legal category for bottom bar CMS pages

Add third placement category for content pages that appear in the
bottom bar alongside the copyright notice (Privacy Policy, Terms, etc.):

Model changes:
- Add show_in_legal boolean field to ContentPage model
- Add to to_dict() serialization

Service changes:
- Add legal_only filter to list_pages_for_vendor()

Platform changes:
- Fetch legal_pages in get_platform_context()
- Update base.html to render legal_pages dynamically
- Fallback to hardcoded links if no CMS pages configured

Migration:
- Add column with default=False
- Auto-set show_in_legal=True for privacy and terms pages

Categories:
- show_in_header: Top navigation
- show_in_footer: Quick Links column
- show_in_legal: Bottom bar with copyright (NEW)

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

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

models/database/content_page.py

@@ -82,6 +82,7 @@ class ContentPage(Base):
     display_order = Column(Integer, default=0)
     show_in_footer = Column(Boolean, default=True)
     show_in_header = Column(Boolean, default=False)
+    show_in_legal = Column(Boolean, default=False)  # Bottom bar with copyright
 
     # Timestamps
     created_at = Column(
@@ -153,6 +154,7 @@ class ContentPage(Base):
             "display_order": self.display_order,
             "show_in_footer": self.show_in_footer,
             "show_in_header": self.show_in_header,
+            "show_in_legal": self.show_in_legal,
             "is_platform_default": self.is_platform_default,
             "is_vendor_override": self.is_vendor_override,
             "created_at": self.created_at.isoformat() if self.created_at else None,