feat: allow organizations to define a default project for all members

[Nithishvb]

Closes: #1173

Summary
Adds the ability for organizations to define a default project. Every org member (existing and future) automatically gets access without an admin adding them one by one.

Changes

• Schema: Added default_project_id column to organization table (SQLite + Postgres migrations included)
• Backend: New query functions setOrgDefaultProject, ensureUserInDefaultProject, ensureAllOrgMembersInDefaultProject
• Auto-enrollment: New SSO users, credential signups, and invited members are all added to the org's default project
• Startup backfill: ensureDefaultProject now calls ensureAllOrgMembersInDefaultProject so existing members never have gaps
• tRPC: organization.getDefaultProject / organization.setDefaultProject routes
• UI: Default Project card in org settings with a project selector dialog (admin only)
• Env var gating: When NAO_DEFAULT_PROJECT_PATH is set in self-hosted, the UI shows a read-only message and the tRPC route rejects changes

[github-actions]

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