Add multi-tenancy with Teams feature

Implement team-based organization for projects with role-based access control:

Backend:
- Add teams and team_memberships database tables (migrations 009, 009b)
- Add Team and TeamMembership ORM models with relationships
- Implement TeamAuthorizationService for team-level access control
- Add team CRUD, membership, and projects API endpoints
- Update project creation to support team assignment

Frontend:
- Add TeamContext for managing team state with localStorage persistence
- Add TeamSelector component for switching between teams
- Add TeamsPage, TeamDashboardPage, TeamSettingsPage, TeamMembersPage
- Add team API client functions
- Update navigation with Teams link

Security:
- Team role hierarchy: owner > admin > member
- Membership checked before system admin fallback
- Self-modification prevention for role changes
- Email visibility restricted to team admins/owners
- Slug validation rejects consecutive hyphens

Tests:
- Unit tests for TeamAuthorizationService
- Integration tests for all team API endpoints

frontend/src/components/Layout.tsx

@@ -77,6 +77,17 @@ function Layout({ children }: LayoutProps) {
               </svg>
               Dashboard
             </Link>
+            {user && (
+              <Link to="/teams" className={location.pathname.startsWith('/teams') ? 'active' : ''}>
+                <svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2">
+                  <path d="M17 21v-2a4 4 0 0 0-4-4H5a4 4 0 0 0-4 4v2"/>
+                  <circle cx="9" cy="7" r="4"/>
+                  <path d="M23 21v-2a4 4 0 0 0-3-3.87"/>
+                  <path d="M16 3.13a4 4 0 0 1 0 7.75"/>
+                </svg>
+                Teams
+              </Link>
+            )}
             <a href="/docs" className="nav-link-muted">
               <svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2">
                 <path d="M14 2H6a2 2 0 0 0-2 2v16a2 2 0 0 0 2 2h12a2 2 0 0 0 2-2V8z"/>