refactor(auth): unify authorization patterns with MANAGE_OWN_PROFILE permission

Issue #2: The profile route used a custom role-based check instead
of the permission-based pattern used everywhere else.

Changes:
- Add MANAGE_OWN_PROFILE permission to backend Permission enum
- Add permission to ROLE_REGULAR role definition
- Update profile routes to use require_permission(MANAGE_OWN_PROFILE)
- Remove custom require_regular_user dependency
- Update frontend Permission constant and profile page
- Update invites page to use permission instead of role check
- Update profile tests with proper permission mocking

This ensures consistent authorization patterns across all routes.

frontend/app/auth-context.tsx

@@ -13,6 +13,7 @@ export const Permission = {
   INCREMENT_COUNTER: "increment_counter",
   USE_SUM: "use_sum",
   VIEW_AUDIT: "view_audit",
+  MANAGE_OWN_PROFILE: "manage_own_profile",
   MANAGE_INVITES: "manage_invites",
   VIEW_OWN_INVITES: "view_own_invites",
   // Booking permissions (regular users)