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/pages/TeamsPage.css

@@ -0,0 +1,218 @@
+.teams-page {
+  padding: 1.5rem 0;
+}
+
+.page-header {
+  display: flex;
+  justify-content: space-between;
+  align-items: flex-start;
+  margin-bottom: 1.5rem;
+  gap: 1rem;
+}
+
+.page-header h1 {
+  margin: 0;
+  font-size: 1.75rem;
+}
+
+.page-subtitle {
+  margin: 0.25rem 0 0;
+  color: var(--color-text-muted);
+  font-size: 0.9375rem;
+}
+
+.team-name-cell {
+  display: flex;
+  flex-direction: column;
+  gap: 0.125rem;
+}
+
+.team-name-link {
+  font-weight: 500;
+  color: var(--color-text);
+  text-decoration: none;
+}
+
+.team-name-link:hover {
+  color: var(--color-primary);
+}
+
+.team-slug {
+  font-size: 0.8125rem;
+  color: var(--color-text-muted);
+}
+
+.team-description {
+  color: var(--color-text-secondary);
+  font-size: 0.875rem;
+  max-width: 300px;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
+/* Empty state */
+.empty-state {
+  text-align: center;
+  padding: 4rem 2rem;
+  background: var(--color-bg-secondary);
+  border-radius: var(--radius-lg);
+  border: 1px solid var(--color-border);
+}
+
+.empty-state svg {
+  color: var(--color-text-muted);
+  margin-bottom: 1rem;
+}
+
+.empty-state h2 {
+  margin: 0 0 0.5rem;
+  font-size: 1.25rem;
+}
+
+.empty-state p {
+  margin: 0 0 1.5rem;
+  color: var(--color-text-muted);
+}
+
+/* Loading state */
+.loading-state {
+  text-align: center;
+  padding: 4rem 2rem;
+  color: var(--color-text-muted);
+}
+
+/* Error message */
+.error-message {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  padding: 0.75rem 1rem;
+  margin-bottom: 1rem;
+  background: var(--color-error-bg, #fef2f2);
+  border: 1px solid var(--color-error-border, #fecaca);
+  border-radius: var(--radius-md);
+  color: var(--color-error, #dc2626);
+  font-size: 0.875rem;
+}
+
+.error-dismiss {
+  background: none;
+  border: none;
+  font-size: 1.25rem;
+  cursor: pointer;
+  color: inherit;
+  padding: 0;
+  line-height: 1;
+}
+
+/* Modal */
+.modal-overlay {
+  position: fixed;
+  top: 0;
+  left: 0;
+  right: 0;
+  bottom: 0;
+  background: rgba(0, 0, 0, 0.5);
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  z-index: 1000;
+  padding: 1rem;
+}
+
+.modal-content {
+  background: var(--color-bg);
+  border-radius: var(--radius-lg);
+  padding: 1.5rem;
+  width: 100%;
+  max-width: 480px;
+  box-shadow: var(--shadow-xl);
+}
+
+.modal-content h2 {
+  margin: 0 0 1.5rem;
+  font-size: 1.25rem;
+}
+
+/* Form */
+.form-group {
+  margin-bottom: 1rem;
+}
+
+.form-group label {
+  display: block;
+  margin-bottom: 0.375rem;
+  font-weight: 500;
+  font-size: 0.875rem;
+}
+
+.form-group input,
+.form-group textarea {
+  width: 100%;
+  padding: 0.5rem 0.75rem;
+  border: 1px solid var(--color-border);
+  border-radius: var(--radius-md);
+  font-size: 0.9375rem;
+  background: var(--color-bg);
+  color: var(--color-text);
+}
+
+.form-group input:focus,
+.form-group textarea:focus {
+  outline: none;
+  border-color: var(--color-primary);
+  box-shadow: 0 0 0 2px var(--color-primary-bg);
+}
+
+.form-hint {
+  display: block;
+  margin-top: 0.25rem;
+  font-size: 0.8125rem;
+  color: var(--color-text-muted);
+}
+
+.form-actions {
+  display: flex;
+  justify-content: flex-end;
+  gap: 0.75rem;
+  margin-top: 1.5rem;
+}
+
+/* Buttons */
+.btn {
+  display: inline-flex;
+  align-items: center;
+  gap: 0.5rem;
+  padding: 0.5rem 1rem;
+  border: none;
+  border-radius: var(--radius-md);
+  font-size: 0.875rem;
+  font-weight: 500;
+  cursor: pointer;
+  transition: all 0.15s ease;
+}
+
+.btn:disabled {
+  opacity: 0.6;
+  cursor: not-allowed;
+}
+
+.btn-primary {
+  background: var(--color-primary);
+  color: white;
+}
+
+.btn-primary:hover:not(:disabled) {
+  background: var(--color-primary-hover);
+}
+
+.btn-secondary {
+  background: var(--color-bg-secondary);
+  color: var(--color-text);
+  border: 1px solid var(--color-border);
+}
+
+.btn-secondary:hover:not(:disabled) {
+  background: var(--color-bg-tertiary);
+}