feat: add link_type field to quick links

Adds link_type column to quick_links table for iframe vs new_tab behavior.
Includes Alembic migration and schema updates.

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

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

src/domains/dashboard/models.py

@@ -31,6 +31,7 @@ class QuickLink(Base):
     # Ordering and display
     position = Column(Integer, default=0)
     is_visible = Column(Boolean, default=True)
+    link_type = Column(String(20), default="iframe")  # "iframe", "new_tab", etc.
 
     # Styling
     color = Column(String(20), nullable=True)  # Hex color for the link card