Fix/saved views ownership idor

[shravanithouta108]

Description

saved_views had no shared flag, so there was no way for teams to publish
views without exposing them to IDOR risk. Added a shared boolean column and
migration so owners can publish views team-wide. Non-owners can read shared
views but cannot modify or delete them (returns 403). Private views remain
invisible to other users entirely.

Related Issues

Closes #1879

Type of Change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• User A cannot see User B's private views
• User A cannot modify or delete User B's views (returns 403)
• Shared views are visible to all users
• Shared views cannot be modified by non-owners (returns 403)
• Owner can modify and delete their own views
• Ran pytest testing/backend -q — all passed

Checklist

• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.